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DAVIS 


GIFT  OF 

PROFESSOR  GEORGE  D. 
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THE  ISOMORPHISM  AND  THERMAL  PROPERTIES 
OF  THE  FELDSPARS. 


PART  I— THERMAL  STUDY, 

ARTHUR  L.  DAY  and  E.  T.  ALLEN. 

PART  II— OPTICAL  STUDY,          J.  P.  1DDINGS. 

WITH  AN  INTRODUCTION  BY 
GEORGE  F.  BECKER. 


WASHINGTON,  D.  C. : 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON. 

1905. 


LIBRARY 

UNIVERSITY  OF  CALIFORNIA 
DAVIS 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  31 


PRESS   OF   GIBSON   BROS. 
WASHINGTON,  D.  C. 


INTRODUCTION. 


BY  GEORGE  F.  BECKER. 


INTRODUCTION. 


The  prime  duty  of  a  geological  survey  is  to  make  a  geological  map 
of  the  country.  Those  who  are  unfamiliar  with  the  duties  of  a  geolo- 
gist are  apt  to  suppose  that  no  great  amount  of  knowledge  is  needed 
to  produce  a  satisfactory  map  of  this  kind.  Those  who  have  tried  it 
know  better.  The  field  geologist  is  at  once  confronted  by  the  theo- 
retical aspects  of  his  science  in  such  a  manner  that  he  is  compelled  to 
adopt  at  least  tentative  views.  He  must  decide  what  is  to  be  mapped, 
and  this  decision  implies  that  he  knows  or  assumes  relations  between 
the  various  members  of  the  series  with  which  he  has  to  do.  All  geolo- 
gists worthy  of  the  name  are  continually  and  painfully  aware  that 
they  deal  largely  in  uncertainties  or  matters  of  opinion,  and  they 
can  not  fairly  be  reproached  with  the  insufficiency  of  the  grounds 
which  they  sometimes  have  to  show  for  the  views  they  adopt,  unless 
they  lay  themselves  open  to  the  accusation  of  neglecting  results 
established  by  theory  and  experiment. 

Geology  is  not  a  science,  but  the  application  of  the  sciences  to  the 
elucidation  of  the  history  of  the  earth.  Its  best  developed  and 
oldest  branch  is  zoological  geology  or  paleontology,  and  next  in  order 
of  development,  though  substantially  the  latest  in  chronological  order, 
is  mineralogical  geology  as  represented  by  petrography.  The  rapid 
advance  in  the  description  of  rocks  is  due,  as  everyone  knows,  to  the 
introduction  of  the  microscope  and  of  exact  optical  methods  in  the 
determinations  of  minerals.  Less  advance  has  been  made  in  the  wider 
subject  called  petrology  or  lithology,  as  well  as  in  orogeny,  vulcanism, 
and  ore  deposits.  The  resources  of  the  terrestrial  laboratory  so  far 
transcend  those  which  can  be  equipped  by  man  that  vast  groups  of 
geological  phenomena  still  await  even  approximate  explanation. 

Observations  on  the  lithosphere  alone  will  not  suffice  to  elucidate 
these  dark  regions.  As  Messrs.  Day  and  Allen  very  properly  insist, 
"geological  field  research  is  a  study  of  natural  end-phenomena,  of 
completed  reactions,  but  with  a  very  imperfect  record  of  the  earlier 
intermediate  steps  in  the  earth-making  processes."  In  fact,  the  un- 
known quantities  outnumber  the  equations  which  field  observation 
puts  at  our  disposal.  In  their  present  state  of  development  the 
sciences  of  physics  and  chemistry  can  aid  the  geologist  only  to  a  mod- 
erate extent.  We  do  not  know  in  most  cases  whether  the  laws  of 

5 


6  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

physics  as  established  under  ordinary  conditions  retain  their  validity 
at  temperatures  exceeding  1000°  C.,  while  of  the  chemical  behavior 
of  substances  at  these  temperatures  chemists  can  tell  us  little  more 
than  that  affinities  are  radically  different  from  those  observed  at  100°. 
Similarly  elasticians  can  discuss  the  small  strains  in  a  building  or  a 
bridge  with  some  approach  to  completeness,  but  they  do  not  even 
make  the  attempt  to  deal  with  deformations  which  are  sensible  to 
the  eye  and  which  are  almost  universal  in  geological  exposures. 

It  was  in  recognition  of  the  need  for  researches  in  physics  which  would 
throw  light  on  geological  problems  that  Dr.  Carl  Barus  was  ap- 
pointed physicist  on  my  staff  in  the  United  States  Geological  Survey 
as  far  back  as  1880,  and  that  a  physical  laboratory  was  estab- 
lished under  that  Survey  in  1882.  This  was  discontinued  in  1892, 
not  because  its  importance  was  underestimated  by  the  Director, 
but  on  account  of  a  failure  of  appropriations.  The  laboratory  was 
reestablished  in  1901  because  it  was  felt  that  without  the  aid  to  be 
derived  from  physical  determinations  the  efficiency  of  the  Survey 
must  suffer.  There  was  nothing  novel  in  the  appreciation  thus  dis- 
played of  the  importance  of  physics  to  geology;  indeed,  several  great 
geophysical  problems  have  been  recognized  by  natural  philosophers 
for  more  than  a  century ;  and  their  difficulty,  not  their  unimportance, 
has  stood  in  the  way  of  experimental  investigation. 

The  field  geologist  meets  with  phenomena  in  all  the  ruggedness  of 
their  utmost  complexity,  and  he  is  sometimes  tempted  to  face  and 
make  an  assault  upon  the  situation  as  he  finds  it.  A  little  consider- 
ation shows  that  in  such  circumstances  a  frontal  attack  must  lead  to 
disaster.  The  outposts  must  be  overcome  one  by  one.  We  must 
patiently  begin  with  the  simplest  problems  that  can  be  devised  and, 
aided  by  the  most  perfect  appliances  known,  study  them  exhaustively 
before  proceeding  to  more  difficult  and  complex  cases. 

In  a  plan  submitted  to  the  Director  when  the  new  physical  labora- 
tory of  the  Survey  was  first  contemplated,  I  laid  especial  stress  upon 
the  study  of  isomorphism  and  eutexia.  These  subjects,  with  the 
determinations  of  thermal  constants  which  they  imply,  have  occu- 
pied the  attention  of  the  physical  laboratory  during  the  greater  part 
of  the  time  since  its  reestablishment,  and  will  continue  to  take  the  first 
place  in  the  researches  there  undertaken. 

It  would  appear  that  the  relations  between  liquids  must  be  reduci- 
ble to  very  general  groups.  Liquids  must  either  be  miscible  or  im- 
miscible, and  miscible  liquids  must  exhibit  either  isomorphic  proper- 
ties or  eutectic  ones.  It  is  possible  that  magmas  are  in  some  cases 


INTRODUCTION.  7 

immiscible ;  thus  zircons  separate  out  in  the  process  of  consolidation 
so  early,  so  completely,  and  in  such  minute  crystals  as  to  suggest 
immiscibility.  On  the  other  hand,  Alexejew  reached  the  conclusion 
that  in  all  cases  where  solutions  do  not  react  upon  one  another  chemi- 
cally, they  become  miscible  above  a  certain  temperature.  Again,  all 
researches  on  the  genesis  of  minerals  from  fused  magmas  show  that, 
as  a  rule,  the  crystals  are  precipitated  from  undercooled  glasses  or 
from  miscible  liquids.  With  some  possible  exceptions,  therefore, 
which,  so  far  as  is  yet  known,  are  unimportant,  the  investigation  of 
liquid  magmas  reduces  to  the  study  of  isomorphous  mixtures  (in 
which  the  physical  properties  are  continuous  functions  of  the  compo- 
sition) and  of  eutectic  ones. 

A  main  aim  of  lithological  studies  for  many  years  has  been  to 
classify  rocks.  It  is  difficult  to  overestimate  the  importance  to  the 
whole  history  of  the  earth  of  a  natural  and  rational  petrological  tax- 
onomy. The  earliest  classifications  were  largely  chemical.  After 
the  introduction  of  the  microscope  they  became  chiefly  mineralogical, 
but  with  Lagorio's  famous  paper  on  the  nature  of  the  glass  base  and 
the  processes  of  crystallization  in  eruptive  magmas,  1887,  chemical 
considerations  again  become  predominant.  In  my  opinion,  classifi- 
cation of  rocks  on  a  basis  of  composition  alone  can  never  be  satis- 
factory or  final.  It  is  possible,  of  course,  to  classify  analyses;  but 
rocks  are  at  least  for  the  most  part  very  variable  mixtures,  without 
analogy  to  definite  chemical  compounds,  and  this  method  thus  fails  to 
cover  the  ground  or  to  reveal  the  relations  of  parts  to  the  whole. 

On  the  other  hand,  physical  chemistry  seems  to  me  to  open  the 
road  to  a  classification  which  must  be  helpful  and  may  possibly  be 
final.  Among  the  ten  or  a  dozen  important  rock-forming  minerals* 


*  In  Bulletin  228  of  the  United  States  Geological  Survey  Professor  Clarke  has 
given  an  estimate,  based  on  nearly  700  analyses,  of  the  approximate  relative 
abundance  of  the  more  important  minerals  found  in  igneous  rocks  and  aggregat- 
ing 94.2  per  cent.  Adding  the  more  important  of  the  minerals  which  eluded 
separate  computation  in  one  sum,  this  table  takes  the  following  form,  which  is 
very  suggestive  with  reference  to  important  silicate  solutions : 

Per  cent. 

Feldspar 59 . 5 

Hornblende  and  pyroxene   16 . 8 

Quartz 12.0 

Biotite 3.8 

Titanium  minerals 1.5 

Apatite 0.6 

Magnetite,  olivine,  leucite,  nepheline,  etc 5.8 

100. O 


8  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

there  can  be  only  a  limited  number  of  isomorphous  series  and  a  limited 
number  of  eutectic  combinations.  The  most  important  isomorphous 
series  forms  the  subject  of  this  publication.  It  is  well  known  to  be 
probable  also  that  the  amphiboles,  the  pyroxenes,  and  the  micas  each 
constitute  isomorphous  groups.  No  other  isomorphous  groups  ap- 
pear to  have  much  lithological  importance.  In  general,  it  is  manifest 
that  isomorphism  is  to  be  expected  only  within  groups  of  closely 
allied  compounds,  and  it  is  even  a  matter  of  surprise  that  the  ortho- 
silicate  anorthite  and  the  polysilicate  albite  should  exhibit  complete 
isomorphism  of  the  simplest  type,  as  Messrs.  Day  and  Allen  have 
shown  that  they  do.  It  seems  hardly  possible,  therefore,  that  a  satis- 
factory classification  of  rocks  can  be  based  on  the  study  of  isomor- 
phous series ;  indeed  the  mineralogical  rock  definitions  of  twenty-five 
years  ago  were  little  else  than  such  a  classification,  which  has  been 
rejected  as  inadequate. 

No  serious  attempt  has  yet  been  made  to  group  rocks  on  eutectic 
principles,  one  very  sufficient  reason  being  our  ignorance  of  eutexia  in 
magmas.  Professor  Lagorio  refers  briefly  to  eutexia,  but  regards  the 
important  solvent  in  magmas  as  a  silicate  of  the  alkalies,  the  glass 
least  subject  to  devitrification.*  Mr.  Teall,  in  1888,  discussed  eutexia 
very  lucidly  and  showed  its  importance  in  the  physics  of  rocks,  f  but 
he  did  not  propose  employing  it  as  a  basis  of  classification.  In  1901 
I  briefly  set  forth  some  of  the  advantages  of  such  a  system.  J 

The  applicability  of  eutexia  to  rock  classification  depends  upon  the 
fact  that  it  makes  the  systematic  discussion  of  magmatic  mixtures 
possible.  Inasmuch  as  the  subject-matter  of  lithology  consists  of 
mixtures,  their  classification  must  be  carried  out  in  terms  of  definite 
or  standard  mixtures,  while  the  only  mixtures  possessing  appropriate 
distinguishing  properties  are  the  eutectics.  Thus  in  dealing  with 
magmas  or  other  heteromorphous  miscible  liquids  the  eutectics  seem 
to  afford  not  only  the  best  but  the  only  natural  and  rational  standards 
of  reference.  With  any  eutectic  as  a  basis,  a  series  of  magmas  may 
be  prepared,  each  differing  from  the  eutectic  by  containing  an  excess 
of  one  or  more  constituents.  Thus  if  abc  represents  an  eutectic  of 
three  substances,  a  mixture  composed  of  a,  mb,  and  nc  may  be  re- 

*  There  is  no  fundamental  difference  between  fluid  solvents  and  solutes,  and  no 
objection  to  regarding  the  alkaline  glass  as  the  solvent  if  found  on  other  grounds 
expedient. 

f  Brit.  Petrography,  1888,  p.  394. 

J  Report  on  the  geology  of  the  Philippine  Islands.  Twenty-first  Ann.  Rep. 
U.  S.  Geol.  Surv.,  Pt.  Ill,  1901,  p.  519. 


INTRODUCTION.  9 

garded  as  the  eutectic  plus  (m  — i)  b  plus  (n  —  i)  c.  In  some  cases 
at  any  rate  the  ground  mass  of  a  rock  (as  Mr.  Teall  pointed  out)  repre- 
sents an  eutectic.  This  is  probably  not  true  in  general  but,  if  it 
were,  the  scheme  proposed  would  be  to  group  together  in  one  genus 
all  the  rocks  which  have  the  same  ground  mass  and  to  regard  the 
phenocrysts  as  minor  or  specific  characteristics.  The  size  of  crystals 
is  no  index  of  the  rapidity  of  their  formation,  and  Messrs.  Day  and 
Allen  have  shown  that  anorthites  of  the  size  of  very  large  pheno- 
crysts may  form  in  a  few  minutes,  while  in  more  viscous  magmas 
small  feldspar  crystals  may  form  with  extreme  slowness.  Hence 
great  care  is  requisite  in  deciding  microscopically  the  question  which 
crystals  were  the  last  to  form. 

It  is  worthy  of  note  that  the  geological  behavior  of  an  intrusive 
or  effusive  rock  is  conditioned  largely  by  the  character  of  the  eutectic. 
So  long  as  this  remains  liquid  the  phenocrysts  are,  mechanically  speak- 
ing, mere  flotsam  and  jetsam  in  the  stream.  The  character  of  the 
eutectic  must  decide  whether  a  lava  pours  down  a  gentle  declivity  as 
does  a  basalt,  or  piles  up  about  the  orifice  like  a  rhyolite.  Now,  if  it  be 
not  essential  to  consider  such  geologically  important  properties  in  the 
classification  of  rocks,  it  is  at  all  events  desirable  to  do  so.  Such 
properties  must  sooner  or  later  be  dealt  with  methodically  by  geolo- 
gists, and  a  thoroughly  rational  classification  of  rocks  will  correlate 
physical  and  chemical  properties. 

These  last  paragraphs  deal  with  plans  rather  than  achievements, 
and  have  been  written  chiefly  to  emphasize  the  importance  of  the 
work  done  by  Messrs.  Day  and  Allen  as  one  step  in  a  broader  scheme. 
Evidently  every  step  of  the  larger  plan  involves  accurate  studies  of 
the  melting  points  and  thermal  properties  of  the  rock-forming  min- 
erals, and  first  of  all  that  most  important  group,  the  lime-soda  feld- 
spars, which  make  up  approximately  one-half  of  the  lithosphere.  In 
the  meantime,  since  my  proposal  to  use  eutectics  as  a  basis  of  rock 
classification  was  printed,  some  valuable  work  has  been  done  on 
eutectics,  chiefly  by  J.  H.  L.  Vogt.* 

Only  the  first  step  has  been  taken  in  this  investigation — the  study  of 
the  triclinic  feldspars  in  dry  fusion.  It  has  been  attended  with  great 
difficulties,  many  of  them  only  touched  upon  in  the  paper  which  fol- 
lows, but  of  which  I  have  been  cognizant  in  detail.  Except  for  Dr. 
Day's  resourcefulness  and  experimental  skill,  success  would  not  have 
been  achieved,  but  a  road  has  now  been  broken  out  in  this  ultra- 


*  Die  Silikatschmelzlosungen.     Christiania,  1903.     Mr.  Vogt   printed   a  pre- 
liminary communication  early  in  1902. 


IO  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

tropical  jungle  which  will  almost  certainly  lead  to  further  successful 
explorations. 

Meantime  the  results  reached  are  of  great  importance.  The 
melting  points  of  the  triclinic  feldspars  have  been  determined  with 
an  accuracy  never  before  attained  in  determinations  at  such  tem- 
peratures. Any  future  correction  of  them  must  be  of  very  trifling 
amount.  These  points,  considered  with  reference  to  composition, 
and  the  very  fine  series  of  specific  gravity  determinations  on  chemi- 
cally pure  feldspars,  seem  to  settle  beyond  question  the  isomorphism 
of  the  plagioclases.  The  first  cogent  arguments  for  this  isomorphism 
were  given  by  Sartorius  von  Walthershausen  in  1853,  but  the  more 
thorough  investigation  of  Tschermak  in  1864  has  properly  connected 
the  theory  with  his  name.  Nevertheless,  some  of  the  ablest  investi- 
gators have  been  unconvinced  that  the  isomorhpism  was  complete, 
and  I  confess  to  surprise  that  the  proof  is  so  irrefragable  as  Messrs. 
Day  and  Allen  have  made  it. 

The  study  of  the  feldspars  and  sodium  tetraborate  (dehydrated 
borax)  have  confirmed  results  of  Professor  Lagorio,  which  are  thus 
summarized  by  Mr.  Teall  (op.  cit.,  p.  397) : 

Silicate  solutions  differ  from  aqueous  solutions  in  the  readiness  with  which  they 
form  amorphous  glass  when  cooled  rapidly.  This  appears  to  be  connected  with 
the  fact  that  they  may  be  readily  overcooled,  and  that  when  in  this  state  they  are 
highly  viscous,  so  that  a  rapid  approach  of  the  molecules  is  prevented.  The  melt- 
ing point  of  glass  is  lower  than  that  of  the  same  substance  in  a  crystalline  condi- 
tion. A  glass,  therefore,  results  from  the  solidification  of  an  overcooled  liquid. 

Messrs.  Day  and  Allen  show  that  crystallization  can  be  brought 
about  at  very  different  degrees  of  overcooling  or  at  very  different 
temperatures,  so  that  the  solidifying  temperature  of  crystals  out  of 
undercooled  liquids  is  not  a  physical  constant,  while  solidification  to 
the  amorphous  state  almost  or  quite  eludes  determination  by  the 
means  found  adequate  to  fix  the  melting  points  of  crystals.* 

*  The  properties  of  amorphous  substances  are  very  perplexing.  It  is  well  known 
that  some  physicists  class  glasses  at  any  temperatures  as  liquids,  and  there  is  no 
question  that  it  is  hard  to  draw  the  line  between  them  and  liquids.  On  the  other 
hand,  Mr.  Spring  has  recently  shown  that  mere  deformation  of  crystalline  metals 
at  ordinary  temperatures  changes  their  densities  and  electrical  potentials,  so  that 
mere  derangement  of  crystalline  particles,  without  any  absorption  of  energy  com- 
parable with  that  accompanying  true  fusion,  suffices  to  impart  to  lead,  silver,  bis- 
muth, etc.,  properties  analogous  to  those  of  glasses.  The  whole  subject  demands 
fuller  investigation  which,  to  be  successful,  must  harmonize  thermal,  electrical,  and 
mechanical  phenomena. 


INTRODUCTION.  1 1 

Mr.  Roozeboom's  discussion  of  isomorphous  mixtures  seems 
admirably  verified  by  this  investigation  of  the  feldspars.  When 
considered  in  connection  with  the  high  viscosity  of  the  materials,  it 
also  explains  the  fact  that  the  curve  of  melting  points  closely  follows 
Krister's  rule.  It  would  seem,  therefore,  that  not  only  concentrated 
solutions  but  isomorphous  ones  form  exceptions  to  the  accepted  laws 
of  dilute  aqueous  solutions.  Such  isomorphous  solutions  as  those  of 
the  feldspars  here  dealt  with  could  in  fact  hardly  be  considered  as 
dilute,  and  some  of  them  (as  for  instance  AbiArii)  must  be  very 
concentrated  solutions,  whichever  of  the  components  is  considered  as 
solvent. 

The  specific  volumes  of  the  feldspars  seem  to  bear  a  relation  to  the 
composition  so  nearly  linear  that  the  differences  may  be  ascribed 
to  unavoidable  errors  in  synthesis  and  analysis.  It  should  not  be 
forgotten,  however,  that  the  specific  volumes  are  determined  at 
something  like  1000°  below  the  temperature  of  crystallization,  and 
that,  since  the  coefficients  of  contraction  of  Ab  and  An  doubtless 
differ  to  some  extent,  variations  in  density  as  determined  at  25° 
might  be  due  not  to  a  lack  of  isomorphism,  but  to  the  difference  in 
contraction  of  the  two  components. 

The  artificial  feldspars  prepared  at  the  cost  of  great  labor  are  pure, 
while  natural  crystals  are  not  so.  Hence  lithologists,  in  making 
separations  by  heavy  solutions,  should  substitute  the  densities  here 
found  for  those  hitherto  employed.  The  changes  are  not  great,  but 
they  are  sufficient  in  some  cases  to  affect  conclusions. 

A  very  noteworthy  result  of  the  investigation  is  the  apparent  super- 
heating of  the  albitic  feldspars.  It  is  pointed  out  by  Messrs.  Day  and 
Allen  that  this  may  be  only  apparent  and  due  to  the  extreme  viscosity 
of  the  melt.  In  fact,  the  separation  of  molecules  in  melting  and  their 
deorientation  must  be  successive  processes,  so  that  in  any  fusion,  if 
the  operation  could  be  instantaneously  arrested,  a  layer  of  molecules 
would  be  found  separated  from  the  solid  mass  but  not  yet  deoriented. 
Such  material  would  differ  from  a  liquid  crystal  by  not  being  in  a 
condition  of  stable  equilibrium. 

Prof.  J.  P.  Iddings  kindly  undertook  the  detailed  examination  of 
the  slides  made  from  the  feldspar  preparations.  He  shows  in  his 
report  that  to  one  per  cent,  or  less,  the  feldspars  correspond  optically 
to  the  mixtures  prepared.  A  closer  correspondence  could  not  be 
hoped  for  in  materials  so  viscous  that  diffusion  afforded  scarcely  any 
aid  in  attaining  homogeneity.  He  has  discussed  many  interesting 
features  of  the  crystallization  of  the  feldspars,  most  of  them  familiar 


12          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

in  effusive  rocks.  Spherulitic  structure  in  particular  is  brilliantly 
illustrated  in  the  slides,  and  in  the  nature  of  the  microlites  of  artificial 
feldspar  he  has  found  nothing  to  suggest  essential  differences  between 
the  experiments  and  natural  processes  the  results  of  which  constantly 
come  under  the  observation  of  lithologists.  Professor  Iddings  has 
also  examined  the  refractive  indices  of  the  artificial  feldspars  and  has 
found  them  accordant  with  isomorphism. 

I  can  not  conclude  this  review  without  a  mention  of  the  tireless 
energy  and  watchfulness  which  Messrs.  Day  and  Allen  have  exercised, 
and  of  which  I  have  been  a  daily  witness,  in  a  most  laborious  task 
attended  by  so  many  difficulties  that  it  sometimes  seemed  almost 
hopeless. 

The  vastness  of  the  field  open  to  geophysical  research  is  partially 
indicated  in  the  preceding  pages,  and  I  have  recently  endeavored  to 
enumerate  somewhat  more  fully  the  pressing  problems  of  geophysics.* 
The  Government  of  the  United  States  has  of  late  years  pursued  the 
enlightened  policy  of  making  yearly  grants  for  chemical  and  physical 
researches  under  the  Geological  Survey,  but  the  appropriations  are 
inadequate  for  the  more  difficult  and  costly  studies  in  this  field. 
This  is  not  strange,  for  though  geophysics  has  already  proved  techni- 
cally valuable,  so  that  mining  engineers  display  a  hearty  interest  in  it, 
and  although  it  will  assuredly  lead  to  the  solution  of  certain  well- 
defined  economic  problems  of  the  first  importance,  its  fundamental 
researches  are  somewhat  remote  from  industry,  and  large  public 
appropriations  are  hardly  to  be  hoped  for  in  the  near  future. 

The  Trustees  of  the  Carnegie  Institution  of  Washington,  recogniz- 
ing these  facts,  have  supplemented  the  Congressional  appropriations 
by  grants  to  Dr.  Day  and  to  myself,  and  the  work  described  in  this 
paper,  though  begun  under  the  Survey,  was  completed  at  the  expense 
of  the  Carnegie  Institution.  In  these  circumstances  the  Director  of 
the  Survey  consented  that  it  should  be  offered  to  the  Institution  for 
publication  with  this  due  recognition  of  the  cooperation  of  the  Survey. 
The  Institution  has  accepted  it  as  its  first  contribution  to  geophysics 
and  defrays  the  cost  of  publication.  It  is  surely  a  matter  of  con- 
gratulation that  the  Government  and  a  private  institution  should 
cooperate  in  the  advancement  of  knowledge.  Such  an  alliance 
brightens  the  prospects  of  Science. 

*  International  Scientific  Congress  of  St.  Louis,  1904,  printed  in  Science,  Octo- 
ber 28,  1904. 


PART  I. 


The  Isomorphism  and  Thermal  Properties 
of  the  Feldspars. 


BY 


ARTHUR  L  DAY  AND  E.  T.  ALLEN. 


THE  ISOMORPHISM    AND   THERMAL  PROPERTIES 
OF  THE    FELDSPARS.* 


The  investigation  here  recorded  is  the  first  chapter  in  a  rather  com- 
prehensive plan  for  the  study  of  the  rock-forming  minerals  at  the 
higher  temperatures.  In  its  broader  outlines,  at  least,  it  is  by  no 
means  a  new  plan.  Mr.  Clarence  King  and  Dr.  George  F.  Becker 
were  inspired  by  a  desire  to  reach  the  mineral  relations  from  the  ex- 
perimental side,  which  is  recorded  in  the  very  earliest  records  of  the 
U.  S.  Geological  Survey,  and  much  of  the  remarkable  ground-breaking 
work  of  Prof.  Carl  Barus  was  undertaken  in  furtherance  of  a  carefully 
prepared  scheme  of  research  along  these  lines.  The  matter  has  been 
advanced  but  little  in  the  intervening  years.  The  present  renewal 
of  the  effort  in  this  direction  is  again  due  to  Dr.  Becker  and  has  had 
the  benefit  of  his  wide  field  experience  and  enthusiastic  and  effective 
cooperation  throughout. 

In  October,  1900,  one  of  the  authors  was  called  from  the  Reichsan- 
stalt  to  equip  a  laboratory  in  the  U.  S.  Geological  Survey  in  which  the 
exact  methods  and  measurements  of  modern  physics  and  physical 
chemistry  should  be  applied  to  the  minerals.  The  ultimate  purpose 
was  geological,  to  furnish  a  better  basis  of  fact  for  the  discussion  of  the 
larger  problems  of  geology,  but  it  appeared  highly  probable  also  that 
a  quantitative  study  of  the  thermal  phenomena  in  this  class  of  sub- 
stances would  offer  new  relations  of  intrinsic  interest  and  of  consider- 
able theoretical  value.  This  inference  has  been  happily  substantiated 
quite  recently  through  the  publication  by  Tammann  of  an  extended 
treatise  on  melting  and  crystallization,  f  in  which  he  offers  some  very 
interesting  speculations  on  the  conditions  of  equilibrium  for  sub- 
stances above  and  below  the  melting  temperature  under  different 
pressures.  The  behavior  of  crystalline  minerals  which  melt  at  tem- 
peratures considerably  higher  than  he  was  able  to  command  offer 
peculiarly  advantageous  opportunities  for  verifying  the  truth  of  his 
inferences  and  of  contributing  further  to  the  knowledge  of  this  most 
important  change  of  state  of  matter. 

*  A  preliminary  paper  containing  the  chief  results  of  this  investigation  was 
read  before  the  Geological  Society  of  Washington,  March  23,  1904,  and  a  brief 
abstract  published  in  Science  (vol.  xix,  p.  734),  May  6,  1904.  A  second  abstract 
appeared  in  the  American  Journal  of  Science  (4),  19,  p.  93,  1905. 

t  Tammann,  "  Krystallisiren  und  Schmelzen."     Leipzig,  1903. 

15 


16          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


TEMPERATURE   MEASUREMENTS. 

It  is  only  a  short  time  since  it  became  possible  to  measure  even 
moderately  high  temperatures  with  certainty  and  to  express  them  in 
terms  of  a  well-established  scale.  Temperature  is  a  peculiar  function 
in  that  it  is  not  additive.  Two  bodies,  each  at  a  temperature  of  50°, 
can  not  be  united  to  obtain  a  temperature  of  100°,  nor  can  any  num- 
ber of  bodies  at  a  temperature  of  50°  or  below  give  us  information 
about  the  temperature  51°  or  above.  Furthermore,  temperature  is 
not  independently  measurable ;  we  can  only  measure  phenomena  like 
the  expansion  of  gases  or  the  conductivity  of  platinum  wire  or  the 
energy  of  thermal  radiation,  which  we  have  good  reason  to  suppose 
will  vary  with  the  temperature  uniformly  or  according  to  a  known 
law. 

The  measure  of  temperature  now  generally  accepted  as  standard  is 
the  expansion  of  hydrogen  gas  between  the  melting  point  of  ice  and 
the  normal  boiling  point  of  water,  divided  into  100  equal  increments 
or  degrees.  Temperatures  above  this  point  have  been  determined  by 
continuing  the  expansion  of  hydrogen  or  nitrogen*  in  the  same  units,  as 
far  as  it  has  been  found  possible  to  provide  satisfactory  containing  ves- 
sels for  the  expanding  gas.  Such  determinations  are  then  rendered  per- 
manent and  available  for  general  use  by  establishing  fixed  points,  such 
as  the  melting  temperatures  of  easily  obtainable  pure  metals,  at  con- 
venient intervals.  Beyond  1 1 50°  no  trustworthy  gas  measurements 
have  been  made,  and  we  have,  therefore,  no  standard  scale.  For 
higher  temperatures  it  is  usual  to  select  some  convenient  phenomenon 
which  is  measurable  up  to  the  temperature  desired,  to  compare  it  with 
the  gas  scale  as  far  as  the  latter  extends,  and  then  to  continue  on  the 
assumption  that  the  law  of  its  apparent  progression  below  1 1 50°  will 
continue  to  hold  above  that  point.  In  this  way  we  obtain  degrees 
which,  if  not  identical  with  the  degrees  of  the  gas  scale,  approximate 
very  closely  to  them,  and  can  receive  a  small  correction  if  necessary, 
whenever  the  gas  scale  shall  be  extended  or  another  scale  substituted. 

The  application  of  measurable  high  pressures  at  the  higher  tem- 
peratures has  never  been  successfully  accomplished,  and  until  some- 
thing can  be  done  in  this  direction,  our  knowledge  of  the  rock-forming 
minerals,  and  in  fact  all  the  generalizations  relating  to  equilibrium 

*  To  600°,  Chappuis  et  Harker,  Travaux  et  raemoires  du  bureau  international 
des  poids  et  mesures,  12,  1902.  To  1 150°,  Holborn  and  Day,  Ann.  der  Physik,  2, 
505,  1900.  English  translation,  Am.  Journ.  Sci.,  (4),  10,  p.  171,  1900. 


GENERAL  PLAN — RELATION  TO  GEOLOGICAL  RESEARCH.  17 

between  the  states  of  matter,  which  have  been  established  for  mode- 
rate temperatures,  must  be  regarded  as  more  or  less  tentative  and 
subject  to  eventual  revision.  We  have  been  accustomed  to  assume, 
both  in  geology  and  in  physics,  with  rather  more  confidence  than 
scientific  experience  justifies,  that  established  relations  for  ordinary 
temperatures  and  pressures  will  hold  in  comparable  ratio  for  the  higher 
temperatures  and  pressures  also.  Experimentation  under  extreme 
conditions  is  slow  and  technically  difficult,  and  it  is,  therefore,  not 
strange  that  simple  relations  which  are  verifiable  within  easily  acces- 
sible conditions  should  now  and  then  be  accorded  the  dignity  of 
natural  laws  without  sufficient  inquiry  into  the  more  remote  con- 
ditions. 

GENERAL   PLAN. 

Our  plan  on  entering  this  field  was  to  study  tne  thermal  behavior 
of  some  of  the  simple  rock-making  minerals  by  a  trustworthy  method, 
then  the  conditions  of  equilibrium  for  simple  combinations  of  these, 
and  thus  to  reach  a  sound  basis  for  the  study  of  rock  formation  or 
differentiation  from  the  magma.  Eventually,  when  we  are  able  to 
vary  the  pressure  with  the  temperature  over  considerable  ranges,  our 
knowledge  of  the  rock-forming  minerals  should  become  sufficient  to 
enable  us  to  classify  many  of  the  earth-making  processes  in  their 
proper  place  with  the  quantitative  physico-chemical  reactions  of  the 
laboratory. 

RELATION  TO  GEOLOGICAL  RESEARCH. 

The  relation  which  this  plan  bears  to  general  geological  research 
may  perhaps  be  expressed  in  this  way.  Geological  field  research  is 
essentially  a  study  of  natural  end-phenomena,  of  completed  reactions, 
with  but  a  very  imperfect  record  of  the  earlier  intermediate  steps  in 
the  earth-making  processes.  The  records  of  the  splendid  laboratory 
experiments  in  rock  synthesis  which  have  already  been  made  are  also 
of  this  character.  The  final  product  has  been  carefully  studied,  but 
the  temperatures  at  which  particular  minerals  have  separated  out  of 
the  artificial  magma,  and  the  conditions  of  equilibrium  before  and 
after  such  separation,  have  not  been*  determined.  In  fact,  except  for 
a  limited  number  of  determinations  of  the  melting  points  of  natural 
minerals,  no  exact  thermal  measurements  upon  minerals  or  cooling 
magmas  have  been  made,  and  it  is  in  this  direction  that  a  beginning 
is  to  be  attempted.  The  temperatures  of  mineral  reactions  under 
atmospheric  pressures  are  nearly  all  within  reach  of  existing  labora- 
tory apparatus  and  methods. 


i8 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


EXISTING  METHODS. 

Furthermore,  the  methods  which  have  been  used  hitherto  in  deter- 
mining these  mineral  melting  points  seem  to  the  authors  to  be  open 
to  serious  objection,  both  in  principle  and  in  application.  They 
depend,  almost  without  exception,  upon  the  personal  judgment  of  the 
observer,  and  not  upon  the  actual  measurement  of  any  physical  con- 
stant. For  this  reason,  perhaps,  more  than  any  other,  the  results 
obtained  by  different  observers  upon  the  same  mineral  from  the  same 
source  do  not  agree  within  considerable  limits,  much  larger  than  can 
be  properly  ascribed  to  impurities  in  the  specimens.  Familiar 
examples  will  best  illustrate  this  point.  Among  the  determinations 
of  the  mineral  melting  points,  two  have  received  much  more  general 
acceptance  than  others — those  of  Joly*  and  of  Doelter.  f 

The  melting  temperatures  which  they  obtained  for  some  of  the 
typical  feldspars  are  as  follows : 


Meldometer 

measurement. 

Thermoelectric 

measurement. 

Joly,  1891. 

Cusack,  1896. 

Gas  furnace. 
Doelter,  1901. 

Electric  furnace. 
Doelter,  1902. 

Microcline  

1175° 

Il6Q° 

IISS° 

I  i  err0 

Albite 

1175 

1  1  72 

I  IO^ 

Oligoclase  
Labradorite  
Anorthite  

1220 
1230 

1235 

IIIO 

1119 

IIIO 

I  I  2O 
1125 
1132 

: 

The  determinations  agree  in  recording  higher  melting  points  toward 
the  calcic  end  of  the  series,  but  the  differences  between  corresponding 
melting  points  by  the  two  methods  is  greater  than  the  observed  differ- 
ences between  different  feldspars. 

Joly's  method  was  novel.  He  stretched  a  thin  strip  of  carefully 
prepared  platinum  foil  between  suitable  clamps,  placed  a  few  grains 
of  the  powdered  mineral  upon  it,  and  mounted  a  small  microscope 
above,  so  as  to  be  readily  trained  on  any  part  of  the  strip.  The  foil 
was  then  heated  by  an  electric  current  which  could  be  very  gradually 
increased,  and  the  temperature  measured  from  the  linear  expansion 
of  the  strip  at  the  moment  when  the  observer  at  the  microscope 
noticed  the  first  signs  of  melting.  The  author  of  this  method  was 
able  to  obtain  concordant  results  with  it  to  within  about  5°  C.,  but 

*  J.  Joly,  Proc.  Royal  Irish  Acad.,  3,  2,  p.  38,  1891.  R.  Cusack,  Proc.  Royal 
Irish  Acad.,  3,  4,  p.  399,  1896. 

t  C.  Doelter,  Tschermak  Min.  u.  Petr.  Mitth.,  20,  p.  210,  1901;  21,  p.  23,  1902. 


DETAILED  PLAN.  19 

differences  several  limes  greater  than  5°  appeared  in  our  observations 
with  the  Joly  apparatus,  unless  the  grains  were  prepared  with  the 
greatest  care  and  all  the  observations  made  by  the  same  observer. 
The  size  and  form  of  the  grains,  the  care  used  in  locating  them 
exactly  in  the  middle  of  the  strip,  every  draught  of  air,  but  most  of 
all  the  judgment  of  the  observer  as  to  when  the  substance  appeared 
to  melt,  all  entered  into  the  result  to  a  very  considerable  degree. 
There  is  also  another  source  of  error  with  which  we  afterward  became 
familiar,  which  may  serve  to  account  for  the  very  large  differences 
between  Joly's  results  and  our  own  later  values  with  some  of  the 
well-known  minerals,  though  not  with  all.  In  certain  of  the  minerals, 
after  melting,  the  resistance  to  change  of  shape,  due  to  viscosity,  is  of 
the  same  order  of  magnitude  as  that  due  to  the  rigidity  of  the  crystal 
just  before  melting,  a  fact  which  may  well  have  led  to  large  errors  of 
judgment  in  this  method  of  detecting  melting  points. 

The  possibility  of  working  very  expeditiously  with  minute  quanti- 
ties of  a  substance  led  us  to  study  this  method  with  great  care,  and  we 
were  fortunate  enough  to  possess  an  instrument  of  Professor  Joly's 
own  model,  made  by  Yeates  &  Son,  Dublin,  but  the  results  obtained 
with  it,  even  under  most  favorable  conditions,  are  more  in  the  nature 
of  personal  estimates  than  of  exact  measurements  of  the  change  of 
state.  Its  value  for  qualitative  study,  and  in  cases  where  only  a  very 
minute  quantity  of  a  substance  is  available,  is  unquestioned. 

Doelter  has  employed  electric  furnaces,  modeled  after  that  in  use  at 
the  Reichsanstalt  by  Holborn  and  Day,  for  the  determination  of  the 
melting  points  of  the  metals.  He  measured  his  temperatures  with 
thermo-elements,  and  used  several  grams  of  material  in  his  determina- 
tions, but  he  also  judged  of  the  approach  of  the  melting  point  by  the 
appearance  of  the  charge  and  usually  recorded  two  temperatures — the 
first  approach  of  viscous  melting  and  the  point  where  the  material 
appeared  to  have  gone  over  into  a  thin  liquid. 

DETAILED   PLAN. 

We  determined  from  the  first  to  get  rid  of  this  personal  factor. 
However  carefully  such  observations  may  be  made,  and  however  well 
supported  by  the  reputation  of  a  particular  scientist  for  skilful  and 
exact  work,  they  can  not  have  a  permanent  value.  Melting  points  of 
pure  minerals  are  not  different,  in  principle  at  least,  from  the  melting 
points  of  other  chemical  compounds  or  of  metals.  They  occur  at  less 
accessible  temperatures  and  involve  some  complicating  phenomena, 
as  we  shall  see  presently,  but  the  change  of  state  of  a  solid  crystalline 


20          ISOMORPHISM  AND  THERMAL  PROPERTIES   OF  FELDSPARS. 

mineral  to  a  liquid  must  of  course  be  defined  by  an  absorption  of  heat. 
Whether  the  appearance  of  the  mineral  charge  in  the  furnace  will 
offer  a  trustworthy  index  through  which  to  locate  this  absorption 
may  well  be  expected  to  differ  with  different  substances.  Nearly  all 
observers  have  recorded  the  fact  that  many  substances  of  this  class 
remain  very  viscous  after  melting,  and  that  the  transition  is  not  well 
marked  in  the  appearance  of  the  material. 

We  therefore  planned  an  apparatus  which  should  be  as  sensitive  as 
possible  to  heat  changes  over  a  long  range  of  temperatures,  and  then 
prepared  to  examine  the  thermal  behavior  of  simple  minerals  of 
natural  or  artificial  composition  when  gradually  heated  or  cooled. 
Changes  of  crystalline  form  (Umwandlungen)  or  of  state  (melting  and 
solidifying)  must  involve  a  more  or  less  sharply  marked  absorption 
or  release  of  heat,  and  be  recorded  as  breaks  in  a  smooth  curve  in  the 
same  way  as  in  the  determination  of  metal  melting  points  or  the 
singularities  of  any  of  the  well-known  chemical  compounds  at  lower 
temperatures. 

APPARATUS. 

The  apparatus  used  in  these  determinations  may  be  assumed  to  be 
fairly  well  known.  It  is  the  same  in  all  essential  particulars  as  that 
used  by  Holborn  and  Day*  in  establishing  the  high-temperature  scale 
with  the  gas  thermometer  at  the  Reichsanstalt.  And  yet  it  is  plain 
that  such  a  scale  requires  some  care  in  the  transplanting,  particularly 
as  the  authors  were  without  a  gas  thermometer  and  were,  therefore, 
not  in  position  to  make  direct  comparisons  with  the  gas  scale. 

THERMO-ELEMENTS. 

The  temperatures  were  measured  with  thermo-elements  exclusively. 
We  obtained  from  Dr.  Heraeus  (Hanau,  Germany)  a  set  of  four  ele- 
ments cut  successively  from  the  same  roll  of  wire,  which,  when  joined 
together,  proved -to  be  identically  alike  in  their  readings  over  the 
range  of  temperatures  covered  by  the  gas  scale  of  the  Reichsanstalt 
(250°  to  1150°  C.)  within  the  limits  of  observation  error.  Through 
the  courtesy  of  Prof.  Holborn  these  were  taken  to  the  Reichsanstalt 
and  measured  in  the  original  melting-point  furnace  with  the  same 
elements  in  terms  of  which  the  gas-thermometer  scale  had  been  ex- 
pressed, and  five  careful  comparisons  made.  These  were  the  melting 
points  of  the  pure  metals,  cadmium  (in  air),  zinc  (in  air),  antimony 
(reducing  atmosphere),  silver  (reducing  atmosphere)  and  copper  (in 

*  Ludwig  Holborn  and  Arthur  L.  Day,  Am.  Journ  Sci  (4),  8,  p.  165,  1899 


TEMPERATURE;  SCALE.  21 

air).  A  fortunate  circumstance  made  it  possible  to  send  these  care- 
fully calibrated  elements  to  Washington  by  messenger,  which  made 
it  certain  that  they  suffered  nothing  in  transit. 

The  elements  were  then  further  compared  in  an  electric  furnace, 
which  will  be  described  below,  and  the  melting  points  of  the  same 
group  of  metals  again  determined  in  our  laboratory.  The  metals 
used,  however,  were  from  other  sources  than  those  which  had  served 
for  the  calibration  at  the  Reichsanstalt.  When  this  test  was  finished, 
we  were  able  to  assure  ourselves  that,  although  all  the  constants  in 
the  measuring  apparatus — thermo-elements,  resistances,  standard 
cells,  metals,  etc. — had  been  changed  in  the  transfer  from  the  Reich- 
sanstalt to  the  Geological  Survey  at  Washington,  the  aggregate  error 
nowhere  exceeded  i°  over  the  entire  range  from  250°  to  1150°.  It 
will  be  remembered  that  i°  was  about  the  accuracy  which  the  stand- 
ard gas  thermometer  showed  at  1000°.  Our  thermo-electric  system 
is,  therefore,  now  doubly  established — (i)  by  direct  comparison  and 
(2)  through  an  independent  series  of  metal  melting  points — upon  the 
gas- thermometer  scale  of  the  Reichsanstalt  within  the  limits  of  error  of 
the  latter,  and  can  be  verified  at  any  time  with  the  help  of  two  of  the 
elements  which  have  been  laid  aside  for  this  purpose,  or  the  melting 
points  of  the  metals.  The  scale  is,  therefore,  permanent. 

TEMPERATURE   SCALE. 

As  the  introduction  of  the  standard  high  temperature  scale  of  the 
Reichsanstalt  into  this  country  and  its  establishment  by  proper 
fixed  points  may  be  a  matter  of  considerable  interest  to  other 
investigators,  some  further  details  regarding  the  metals  chosen  for 
these  fixed  points  are  added  here.  We  tried  to  find  metals  which 
should  not  only  be  of  the  purity  necessary  for  such  standards,  but 
which  should  be  easily  obtainable  in  uniform  quality.  With  four  of 
the  five  metals  of  the  Reichsanstalt  series — cadmium,  zinc,  silver,  and 
copper — no  difficulty  was  experienced,  but  we  were  not  able  to  find 
satisfactory  antimony  in  this  country.  This  need  not  prove  an 
obstacle,  for  the  four  points  mentioned  will  serve  most  purposes  with- 
out a  fifth,  while  if  the  needs  of  an  experiment  are  so  exacting  as  to 
require  an  intermediate  melting  point,  antimony  can  be  imported 
from  Kahlbaum,  of  Berlin,  without  great  delay  or  excessive  cost,  in 
the  same  purity  as  that  originally  used  at  the  Reichsanstalt. 

The  cadmium  and  zinc  in  our  series  were  taken  from  the  regular 
listed  chemicals  of  Eimer  &  Amend  (zinc,  "C.  P.  in  sticks ;"  "cadmium, 
metal  sticks").  The  silver  was  the  well-known  test  silver  of  the 


22 


ISOMORPHISM   AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


Philadelphia  Mint  laboratory,  while  the  copper  was  also  from  Eimer  & 
Amend  ("C.  P.  copper  drops,  cooled  in  hydrogen"). 

Careful  analyses  of  samples  of  the  cadmium,  zinc  and  copper  follow : 


ZINC. 

(Eimer  &  Amend  's  "C.  P. 
in  sticks.") 

CADMIUM. 
(Eimer  &  Amend  's  in 
sticks.) 

COPPER. 
(Eimer  &  Amend  's  "C.  P. 
drops,  cooled  in  hydrogen.") 

Per  cent. 
As                               None 

Per  cent. 
As                             None 

Per  cent. 
Te  and  Se     .            None 

Cu             .  .              Trace 

Cu                      .     Trace 

Sb  Trace 

Pb   .              .         o  0412 

Pb                    .     o  0860 

As  None 

Cd  o  002  1 

Zn  .                 ...      Trace 

Bi  None 

Fe  0.0053 
Co  and  Ni  None 

P'e  0.0025 
Co  and  Ni  None 

Ag  0.018 
Pb  o.ooi 

S                             o  0005 

S                            o  0005 

Co  and  Ni                 None 

Zn  o.oio 

Fe                           *o  011 

Total  impurities..  0.049  1 

Total  impurities  o  .  0890 

Total  impurities  .    o  .  040 

*  This  figure  is  doubtless  somewhat  too  high. 

It  was  not  deemed  necessary  to  make  an  analysis  of  the  silver,  as 
we  were  assured  that  it  contained  no  impurity  which  could  be  quanti- 
tatively determined. 

The  melting  temperatures  of  cadmium  and  zinc  are  relatively  low, 
and  those  of  silver  and  copper  comparatively  high  on  the  gas  scale, 
with  a  long  interval  between,  so  that  it  sometimes  becomes  very 
desirable  to  have  an  intermediate  point.  The  two  melting  points 
which  are  most  conveniently  located  are  aluminium  and  antimony. 
Aluminium,  on  account  of  its  low  density,  and  perhaps  because  it 
has  been  less  successfully  purified  than  the  other  metals,  does  not 
give  a  sharp  and  satisfactory  melting  point.  The  melting  point  of 
Kahlbaum's  antimony,  of  which  a  recently  published  analysis  is 
reproduced  here,  serves  this  purpose  excellently.  It  rarely  solidifies 
without  considerable  undercooling,  but  the  point  to  which  the  tem- 
perature rises  after  crystallization  begins  is  sensibly  identical  with 
the  melting  point. 


ANTIMONY  (KAHLBAUM,  BERLIN). f 

Fe 0.012% 

004 

003 


Cu 
Pb 


.019% 


The  C.  P.  antimony  obtained  from  Eimer   &  Amend    and   from 
Merck  &  Co.,  in  a  careful  analysis,  for  which  we  are  indebted  to  Dr. 


f  Fritz  Henz,  Inaugural  Dissertation,  Zurich.     Published  Leipzig,  1903. 


TEMPERATURE  SCALE. 


W.  F.  Hillebrand  of  the  Geological  Survey,  each  showed  about  one  per 
cent  of  the  sulphide  still  present  and  traces  of  other  impurities.  The 
melting  temperatures  of  these  varied  under  different  conditions  as 
much  as  1 5°,  and  were  totally  unsuited  to  this  work. 

Inasmuch  as  the  melting  points  of  these  metals  were  determined 
with  thermo-elements  which  Professor  Holborn  had  just  calibrated 
with  the  metals  in  use  at  the  Reichsanstalt  for  this  purpose,  a  compari- 
son of  the  values  obtained  will  show  the  accuracy  with  which  one  may 
reproduce  the  Reichsanstalt  scale  entirely  from  local  sources: 


Metal. 

Reichsanstalt. 

Day  and  Allen. 

Differ- 
ence. 

Source. 

Melting 
point. 

Source. 

Melting 
point. 

Cadmium..  .  . 
Zinc  

Silver 

Kahlbaum    . 

321.7° 
419.0 

96i.5 
1064.9 
1084.1 

Eimer  &  Amend 
Do. 
Philadelphia  Mint 

Eimer  &  Amend 
Do. 

321-7° 
420.0 
962.  2 

1065.3 

*io83.6 

0.0° 
I  .0 

0.7 
0.4 
0.5 

Do. 
Gold  u.  Silber 
Scheideanstalt. 

Haddernheim 
Kupferwerk. 

Do. 

Copper  (in 
air)  

Copper  (re- 
ducing at- 
mosphere.) 

*  A  single  determination  with  one  element;  all  others  are  mean  values  with  two 
or  more  elements. 

For  the  method  of  extrapolation  of  the  scale  and  further  informa- 
tion regarding  the  use  and  accuracy  of  thermo-elements  at  these 
temperatures,  reference  is  made  to  the  papers  of  Holborn  and  Day 
already  cited. 

For  everyday  use,  four  more  elements  were  prepared  and  calibrated 
in  the  same  way.  Of  these,  two  are  of  the  usual  form  (fig,  i)  and  two 
are  of  a  new  design  which  has  proved  very  effective  in  the  determi- 
nation of  the  melting  points  of  non-metallic  substances.  It  will  be 
seen  from  the  diagram  of  the  insulated  element  that  the  hot  junction 
is  protected  from  the  melting  charge  by  a  casing  of  platin-iridium 
(0.5  mm.  thick)  and  by  a  protecting  tube  of  refractory  Berlin  (Mar- 
quardt)  porcelain  (1.5  mm.  thick) .  Very  early  in  our  experiments  upon 
the  mineral  silicates  we  became  aware  that  the  conductivity  of  these 
materials  for  heat  would  be  much  poorer  than  in  similar  charges  of 
metal.  Furthermore,  the  charge  of  mineral  which  the  furnaces  could 
carry  was  only  one-fourth  to  one-third  as  great  as  the  metal  charges 
used  in  the  calibrations,  because  of  the  great  difference  in  specific 
gravity  and  the  limited  space  which  could  be  heated  to  a  fairly  uni- 
form temperature.  For  these  reasons  the  changes  of  state  would  be 
less  sharply  marked  upon  the  heating  and  cooling  curves  than  metal 
melting  points,  and  it  was  feared  that  the  readings  of  the  protected 


Pt 


PtRhX 


24          ISOMORPHISM  AND  THERMAL  PROPERTIES  OP  FELDSPARS. 

element  might  prove  too  high  or  too  low  through  inability  to  take  on 
the  temperature  of  the  surrounding  mass  promptly.  It  was  to  dis- 
cover and  obviate  this  possible  source  of  error  that  the  form  of  thermo- 
element indicated  in  the  adjoining  diagram  (fig.  2)  was  devised.  It 
really  amounts  to  nothing  more  than  the  ordinary 
form  of  platinum — platin-rhodium  element  with  the 
platinum  wire  insulated  from  the  other  by  a  very 
slender  porcelain  (Marquardt)  tube  and  the  platin- 
rhodium  wire  broadened  out  and  wrapped  around 
this  tube  like  a  cap  over  the  portion  which  dips  into 
the  charge.  The  hot  junction  is 
then  the  lower  extremity  of  the  cap 
where  the  platinum  wire  emerges 
from  its  insulating  tube  and  is  welded 
inside  the  platin-rhodium  cap. 

This  form  was  dictated  entirely  by 
experience  to  meet  conditions  where 
an  exposed  element  might  be  neces- 
sary or  desirable.  The  wires  of  an 
ordinary  element,  if  embedded  with- 
out protection,  are  rather  frail  for  the 
wear  and  tear  of  breaking  or  drilling 
mineral  charges  out  of  the  crucibles 
after  the  measurements,  and  they  cau 
not  be  strengthened  without  intro 
ducing  a  greater  error  through  the 
amount  of  heat  conducted  away  from 
the  junction  than  the  one  which  it  is 
desired  to  obviate. 

It  has  furthermore  been  the  almost 
invariable  experience  of  one  of  the 
authors*  that  when  an  element, 
through  exposure  to  combustion  products  or  otherwise,  no  longer 
gives  normal  readings,  the  seat  of  the  trouble  lies  in  the  5  or  6  centi- 
meters of  the  platinum  wire  immediately  adjacent  to  the  hot  junc- 
tion, and  not  in  the  alloy.  The  pure  platinum  sometimes  seems  to 
absorb  enough  volatile  or  other  contact  products,  when  unprotected  in 
a  furnace  at  very  high  temperatures,  to  alter  both  its  resistance  and 
its  thermo-electric  potential. f  Changes  of  this  kind  are  not  serious 
when  a  number  of  control  elements  are  constantly  available,  and  they 


I.  2. 

FIG.  i. — Thermoelement  (standard 

form)  in  position. 
FIG.  2. — A  new  form  of  thermo-ele- 

ment. 


Day. 


t  Holborn  &  Day,  Am.  Journ.  Sci.  (4),  10,  p.  171,  1900. 


FURNACE.  25 

are  usually  permanently  corrected  by  half  an  hour's  glowing  in  the  open 
air  at  full  white  heat.  The  glowing  must  be  done  by  passing  a  suitable 
current  through  from  end  to  end  and  not  with  a  Bunsen  burner  or 
gas  blast. 

In  event  of  a  serious  accident  involving  an  exposure  of  the  element 
which  can  not  be  corrected  by  glowing,  cutting  out  the  exposed  por- 
tion of  the  platinum  wire  and  reconnecting  will  almost  always  restore 
the  normal  readings. 

The  new  form  of  element  seeks  to  avoid  both  these  difficulties;  it 
offers  the  advantage  of  an  exposed  junction  without  exposing  the 
platinum  wire,  and  by  making  the  platin-rhodium  cap  project  but 
little  above  the  surface  of  the  melting  charge  it  avoids  excessive  loss 
of  heat  by  conduction  away  from  the  hot  junction.  In  fact,  in  this 
latter  particular  the  new  form  enjoys  a  distinct  advantage  over  the 
usual  form  of  heavily  protected  element.  It  has  the  disadvantage  of 
being  more  frail  to  handle,  but  there  is  little  danger  of  r*  nything  more 
serious  happening  than  the  breaking  of  the  porcelain  tube,  which  is 
readily  replaced. 

These  elements  are  calibrated  in  metal  baths  like  the  others  by 
inclosing  in  a  porcelain  protecting  tube. 

It  need  only  be  added  that  all  the  more  important  temperatures 
throughout  this  work  were  separately  determined  with  three  different 
elements.  One  of  these  was  always  from  the  Reichsanstalt  set  (pro- 
tected) and  one  usually  an  unprotected  element  of  the  new  form. 
No  systematic  differences  between  the  readings  of  the  two  types 
have  ever  been  found. 

FURNACE. 

The  furnace,  in  plan,  differed  but  little  from  that  in  use  for  melting- 
point  determinations  at  the  Reichsanstalt.  In  the  working  out,  two 
important  changes  were  introduced,  in  order  to  enable  it  to  reach  the 
higher  temperatures  of  the  mineral  melting  points.  A  more  refrac- 
tory and  better  insulating  material  (a  mixture  of  magnesite  and 
corundum)  was  substituted  for  fire  clay  in  the  hotter  parts  and  the 
coil  was  wound  on  the  inside  of  the  oven  tube  instead  of  outside.  The 
latter  involved  some  little  mechanical  ingenuity  and  skill  in  winding, 
but  the  gain  in  economy  and.in  the  rapidity  with  which  changes  could 
be  effected  or  constant  conditions  established  more  than  repaid  any 
additional  labor  in  preparation. 

A  diagram  of  the  furnace  in  section  is  shown  in  fig.  3.  It  could  be 
used  for  any  temperature  up  to  1600°  C.  without  any  difficulty  or 
especial  precautions  and  could  be  regulated  to  maintain  a  constant 
temperature  at  a  particular  point  for  long  periods  of  time. 


Ptw 


26  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

The   coil,  which  was  obtained  from  Dr.  Heraeus,  was  of  platin- 
iridium  wire  (90  parts  Pt.,  10  parts  Ir.),  1.5  mm-  in  diameter,  and 

required  about  3000  watts  to 
maintain  a  constant  temperature 
of  1600°  C.  The  furnace  was 
carried  at  times  on  a  no- volt 
direct-current  street  main,  but 
accurately  constant  temperatures 
could  not  be  depended  on  without 
the  storage  battery. 

The  insulation  in  these  furnaces 
was  so  perfect  that  shutting  off  or 
reversing  the  heating  current  at 
the  highest  temperatures  did  not 
produce  a  quiver  in  the  galvanom- 
eter to  which  the  thermo-element 
was  connected,  although  the  sen- 
sitiveness of  the  system  was  such 
that  a  leakage  amounting  to  a 
single  micro-volt  (corresponding  to 


FIG.  3. — The  furnace,  showing  ther- 
mo-element and  charge. 


less  than  0.1°)  at  1600°  would  have  caused  a  displacement  of  more 
than  two  millimeters  on  the  scale. 

STANDARDS. 

The  thermo-electrical  potential  was  measured  upon  a  potentiom- 
eter (Wolff,  Berlin,  Reichsanstalt  calibration)  in  terms  of  a  standard 
cadmium  cell  (saturated)  prepared  by  ourselves.  Two  of  these 
cells  were  used  interchangeably  during  the  earlier  measurements. 
Toward  the  close  of  the  series  four  fresh  cells  were  prepared  for  com- 
parison with  the  earlier  ones  and  were  found  to  agree  with  them  within 
o.ooo i.  One  of  these  later  cells  (the  readings  of  the  four  were  iden- 
tical to  the  fifth  significant  figure)  was  verified  by  Dr.  Wolff,  of  the 
Bureau  of  Standards,  by  comparison  with  the  standard  Clark  cells  of 
that  institution  and  found  to  be  1.0195  V  at  20°  C.,  assuming  the  legal 
value  (United  States)  of  the  Clark  cell,  1.434  V,  at  15°  C.  Substitut- 
ing the  Reichsanstalt  value,  Clarki5  =  1.4328,*  our  cells  would  give 
a  normal  potential  difference  of  1.0186  at  20°.  The  temperature 
determinations  which  follow  are,  therefore,  calculated  in  terms  of  this 
number. 

With  the  apparatus  here  described,  the  authors  were  enabled  to 
command  any  furnace  temperature  up  to  1600°  conveniently,  to  regu- 


*  Jaeger  u.  Kahle,  Wied.  Ann.,  65,  p.  926,  1898. 


FIRST  GROUP  OF  MINERALS  INVESTIGATED.  27 

late  it  quickly  and  with  great  exactness,  or  to  hold  it  constant  for  long 
intervals.  An  oxidizing  or  reducing  atmosphere  could  also  be  easily 
introduced  whenever  desired.  It  is,  however,  undesirable  to  expose 
either  coil  or  thermo-element  too  freely  to  oxygen  at  very  high  tem- 
peratures on  account  of  the  considerable  losses  by  sublimation  to 
which  the  platinum  metals  are  subject. 

With  the  help  of  the  standard  metals  mentioned,  which  are  readily 
obtainable  andean  be  used  repeatedly,  thermo-elements  or  resistance 
pyrometers  can  be  calibrated  in  any  laboratory,  and  used  for  all 
measurements  up  to  the  limit  of  the  Reichsanstalt  scale  (i  1 50°  C.)  with 
no  greater  error  than  that  inherent  in  the  scale  itself.  Above  this 
temperature  up  to  1600°  the  continuation  of  the  thermo-electric  scale 
probably  still  furnishes  the  most  convenient  and  trustworthy  extra- 
polation which  has  yet  been  perfected. 

The  uniformity  and  certainty  of  this  extrapolation  will  best  be 
illustrated  by  the  measurements  upon  anorthite  (the  highest  melting 
point  we  measured).  The  melting  temperature  of  a  mineral  of  very 
poor  conductivity  for  heat  and  relatively  low  specific  gravity  is  much 
more  difficult  to  measure  than  that  of  a  metal,  but  the  agreement  of 
the  results  tabulated  below  (see  Anorthite,  p.  37)  is  sufficiently  good 
to  demonstrate  the  accuracy  of  the  extrapolation.  The  thermo- 
electric potential,  therefore,  appears  to  deserve  entire  confidence  for 
consistent  extrapolation  through  the  450°  immediately  above  the 
present  Reichsanstalt  scale. 

FIRST   GROUP   OF   MINERALS   INVESTIGATED. 

The  particular  group  of  minerals  chosen  for  the  first  investigation 
was  the  soda-lime  feldspar  series,  and  orthoclase.  The  reasons  for 
this  choice  will  be  fairly  obvious.  Aside  from  its  being  altogether  the 
most  important  group  of  rock-forming  minerals,  unusual  interest  has 
been  attracted  to  it  through  Tschermak's  theory  that  these  feldspars 
bear  a  very  simple  relation  to  one  another,  that  they  are  (orthoclase 
excepted,  of  course)  in  fact  merely  isomorphous  mixtures  of  albite 
and  anorthite.  This  hypothesis  has  given  occasion  for  serious  and 
extended  study,  both  from  the  optical  and  thermal  sides. 

A  complete  review  of  the  literature  of  the  feldspars  will  not  be 
attempted  here.  Although  opinion  is  still  somewhat  divided,*  it  is 
probably  fair  to  say  that  the  optical  researches  have  not  yet  definitely 
established  or  disestablished  the  isomorphism  of  the  albite-anorthite 

*  Fouque  et  Levy,  Synthesedes  Mineraux  et  des  Roches,  p.  145,  1882;  C.  Viola, 
Tschermak  Min.  &  Petr.  Mitth.,  20,  p.  199,  1901;  Lane,  Journ.  Geol.,  XII,  2,  p. 
83,  1904;  J.  H.  L.  Vogt,  "  Die  Silikatschmelzlosungen,"  Christiania,  1903. 


28  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

group.  Investigation  from  the  thermal  point  of  view  has  been  even 
less  satisfactory  by  reason  of  the  subjective  methods  employed,  to 
which  reference  has  already  been  made,  though  the  recorded  results 
indicate  with  reasonable  unanimity  that  the  melting  point  of  anorthite 
is  above  that  of  albite  and  that  the  intermediate  feldspars  will  prob- 
ably fall  between  the  two.*  Beyond  this  conclusion,  the  great  body 
of  evidence  is  more  or  less  contradictory  and  sometimes  contro- 
versial in  character. 

ORTHOCLASE  (PRELIMINARY). 

Somewhat  unluckily,  our  measurements  began  with  natural  ortho- 
clase  (microcline)  from  Mitchell  County,  North  Carolina,  a  quantity 
of  which  was  placed  at  our  disposal  by  the  U.  S.  National  Museum. 
The  material  was  powdered  so  as  to  pass  readily  through  a  loo-mesh 
sieve,  and  placed  in  100  cc.  or  125  cc.  platinum  crucibles,  sometimes 
open  and  sometimes  covered,  in  charges  of  from  100  to  150  grams. 
These  charges  were  heated  slowly  in  the  electric  furnace  from  600°  to 
above  1400°  C.,  but,  although  the  thermal  apparatus  was  sufficiently 
sensitive  to  detect  an  unsteadiness  of  a  tenth  of  a  degree  with  certainty 
not  the  slightest  trace  of  an  absorption  or  release  of  heat  was  found. 
The  charge  at  the  beginning  of  the  heating  was  a  dry  crystalline 
powder  which  was  prodded  from  time  to  time  with  a  stout  platinum 
wire  to  ascertain  its  condition  as  the  heating  progressed.  At  about 
1 000°  traces  of  sintering  were  evident;  at  1075°  it  had  formed  a  solid 
cake  which  resisted  the  wire,  at  1150°  this  cake  had  softened  suffi- 
ciently to  yield  to  continued  pressure,  and  at  1300°  it  had  become  a 
viscous  liquid  which  could  be  drawn  out  in  glassy,  almost  opaque 
threads  by  the  wire.  Under  the  microscope  the  opacity  was  seen  to 
be  due  to  fine  included  bubbles,  the  material  being  entirely  vitreous. 
The  cooling  was  equally  uninstructive ;  the  vitreous  mass  solidified 
gradually  without  recrystallization  or  the  appearance  of  any  thermal 
phenomenon.  Frequent  repetitions  with  fresh  charges  and  varied 
conditions  added  nothing  to  our  knowledge  of  the  melting  tempera- 
ture, and  the  matter  began  to  look  very  unpromising. 

We  also  reheated  charges  of  the  resulting  glass,  which  was  some- 
times repowdered  and  sometimes  in  the  cake  as  it  had  cooled.  But 
except  to  observe  that  the  glass  powder  began  to  sinter  earlier  (800°), 
no  new  facts  appeared,  f 

*  J.  H.  L.  Vogt,  loc.  tit.,  p.  154,  expresses  the  opinion  that  the  soda-lime  feld- 
spars fall  under  Type  III  of  Roozeboom's  types  of  isomorphous  series  with  a 
minimum  between  anorthite  and  albite. 

t  These  sintering  temperatures  varied  within  considerable  limits  with  the  fine- 
ness of  the  material  and,  therefore  serve  only  in  a  very  rough  way  to  define  the 
state  of  the  charges. 


BORAX.  29 

Then  we  tried  by  various  means  to  recrystallize  the  melted  ortho- 
clase.  We  mixed  crystalline  powder  with  the  glass,  we  applied  suc- 
cessive quick  shocks  to  the  cooling  liquid  for  several  hours  with  an 
electric  hammer  below  the  crucible,  we  varied  the  rate  of  cooling  and 
even  tried  rapid  see-sawing  between  800°  and  1300°.  We  circulated 
air,  water  vapor*  and  carbonic  dioxide  through  the  charge  throughout 
the  heating,  and  finally  introduced  a  rapid  alternating  current  sent 
directly  through  the  substance  while  cooling,  but  no  trace  of  crystalli- 
zation resulted.  An  extremely  viscous,  inert  mass  always  remained, 
which  gradually  hardened  into  a  more  or  less  opaque  glass.  It  ap- 
peared somewhat  translucent  if  very  high  temperatures  had  been 
reached,  but  was  never  clear. 

Following  orthoclase,  a  number  of  specimens  of  natural  albite  were 
tried  under  similar  conditions  and  with  entirely  similar  results. 

Later  on,  when  more  experience  had  been  acquired,  these  minerals 
were  taken  up  again  and  a  satisfactory  explanation  for  their  behavior 
was  found.  But  for  the  moment  all  the  defining  phenomena  ap- 
peared to  be  so  effectively  veiled  by  some  property,  presumably  the 
viscosity,  that  we  were  constrained  to  look  about  for  some  similar 
compound  which  should  give  us  a  better  insight  into  the  behavior  of 
mineral  glasses  and  their  thermal  relations,  and  to  lay  aside  the  feld- 
spars until  they  could  be  more  successfully  handled. 

This  outline  of  our  unsuccessful  experiences  is  given  here  in  some 
detail,  in  order  to  show  the  actual  difficulties  which  confront  the 
student  in  working  with  the  feldspars,  in  the  face  of  which  it  is  cer- 
tainly not  surprising  that  uncertain  and  contradictory  conclusions 
have  been  reached. 

BORAX. 

The  substance  chosen  for  this  preliminary  work  was  ordinary 
anhydrous  borax  (sodium  tetraborate) .  We  chose  this  merely  be- 
cause it  was  a  simple  glass  and  unlikely  to  undergo  chemical  change. 
It  is  easily  obtainable  pure  and  its  thermal  phenomena  are  within 
easy  reach.  The  study  of  borax  proved  to  be  most  instructive. 
It  gave  us  an  effective  insight  into  the  behavior  of  this  class  of  sub- 
stances, and  in  particular  served  to  define  the  phenomena  of  melt- 
ing and  solidifying  in  substances  which  undergo  extreme  under- 
cooling and  which  recrystallize  with  difficulty,  or  not  at  all.  The 
results  of  this  study  of  borax  were,  therefore,  of  much  interest  in  them- 
selves and  were  given  in  a  paper  before  the  National  Academy  of 
Sciences  at  its  spring  meeting  in  Washington  last  year  (April  21,  1903), 
but  were  not  printed  at  that  time. 


30  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

The  borax  glass  upon  which  our  measurements  were  made  was 
prepared  in  the  usual  way  by  heating  the  crystals  until  the  water  of 
crystallization  had  been  driven  off  and  the  viscous  mass  was  reason- 
ably free  from  bubbles.  If  the  borax  is  pure,  the  anhydrous  product, 
when  cooled,  is  a  brilliant,  colorless  glass,  isotrophic,  of  conchoidal 
fracture,  and  specific  gravity  2.37.  The  specific  gravity  was  deter- 
mined in  the  fraction  of  kerosene  boiling  above  1 85°  C.  About  100  g. 
of  this  glass  were  then  broken  up  and  placed  in  a  platinum  crucible 
in  the  electric  furnace.  The  thermo-element  was  placed  in  position 
as  indicated  in  fig.  3,  the  heating  current  properly  regulated,  and  ob- 
servations of  the  temperature  made  at  intervals  of  one  minute,  while 
the  glass  softened  and  passed  gradually  over  into  a  thin  liquid  (800°). 
Then  the  current  was  reduced  and  the  cooling  curve  observed  in  the 
same  way.  These  observations  gave  an  unbroken  curve,  both  for  the 
heating  and  cooling,  as  in  the  case  of  all  the  glasses,*  without  a  definite 
melting  or  solidifying  point,  although  the  arrangements  for  detecting 
an  absorption  or  release  of  heat  were  very  sensitive.  Prodding  at  in- 
tervals with  a  platinum  rod  showed  the  change  to  be  perfectly  gradual 
from  a  clear,  hard  cake  through  all  degrees  of  viscosity  to  a  fairly  thin 
liquid  and  back  again.  This  observation  is  of  considerable  interest 
as  showing  that  the  absence  of  bounding  phenomena  between  the  cold 
glass,  which  fulfills  the  mechanical  conditions  for  a  solid  very  perfectly, 
and  the  liquid,  is  not  confined  to  mixtures  of  complicated  chemical 
composition,  but  is  exhibited  also  by  true  chemical  compounds  of 
undoubted  purity.  It  is,  therefore,  not  conditioned  by  composition, 
but  by  the  physical  nature  of  the  substance. 

Having  verified  this  behavior  of  anhydrous  borax  by  several  repeti- 
tions of  the  experiment,  various  disturbing  influences  were  applied  to 
the  slowly  cooling  liquid  in  the  hope  that  some  temperature  or  range 
of  temperature  would  be  found  within  which  the  vitreous  condition 
would  prove  unstable  and  crystallization  be  precipitated.  The  jar 
produced  by  an  electric  hammer  pounding  upon  the  outside  of  the 
furnace  during  cooling  proved  to  be  sufficient  to  bring  down  the  entire 
charge  as  a  beautiful  crystalline  mass  of  radial,  fibrous  structure, 
brilliant  luster,  rather  high  refractive  index,  and  increased  volume. 
Fig.  4  will  give  a  good  idea  of  the  appearance  of  the  anhydrous  crys- 
talline borax  in  the  crucible.  Its  specific  gravity  proved  to  be  2.28 
as  compared  with  2.37  for  the  glass,  a  somewhat  unusual  relation, f 
which  may,  in  part,  account  for  the  quasi  stability  of  the  vitreous  form 
during  cooling. 


*  See  Tammann,  loc.  tit.;  also  Roozeboom,  "Die  heterogenen  Gleichgewichte, 
etc.,"  Braunschweig,  1901. 

*  Tammann,  loc.  cit.,  p.  47  et  seq. 


FIG.  4. 

CHEMICALLY  PURE   BORAX   (ANHYDROUS), 
CRYSTALLIZED    FROM   THE  GLASS. 

Heliotype   CD.,   Boston, 


ui  mourns  wiiicu  uiiueicuui  in  boiiuiiying.     we  next  vaneQ  tne  experi- 
ment by  first  cooling  quietly  to  about  100°  below  the  melting  point 


*  See  Tammann,  loc.  tit.;  also  Roozeboom,  "Die  heterogenen  Gleichgewichte, 
etc.,"  Braunschweig,  1901. 

•*•  Tammann,  loc.  tit.,  p.  47  et  seq. 


BORAX. 


Observations  were  then  undertaken  upon  the  crystalline  borax  with 
a  thermo -element  as  before,  to  determine  the  melting  temperature  and 
solid  modifications,  if  such  existed,  but  none  of  the  latter  were  found. 
The  charge  melted  uniformly  at  742°  and  the  melting  point  was  well 
defined.  A  curve  showing  the  minute-to-minute  observations  on  the 
crystalline  borax  between  the  temperatures  650°  and  775°  is  shown  in 
fig.  5,  a. 

Having  determined  the  melting  point  of  crystalline  anhydrous 
borax  satisfactorily,  we  examined  more  closely  into  the  conditions 


7000 


6900 

(765") 


6800 
6700 


6600 

(MO*) 

6500 


6400 
6300 
6200 
6100 
6000 
5900 


-\ 


5800 

(660*) 


5700 


Time    -    I   divisior 


10  minutes 


FlG.  5.- 


i,  Melting-point  curve;  b,  c,  d,  curves  showing  undercooling  and 
crystallization  at  different  temperatures. 


under  which  it  solidified.  As  has  been  said,  if  the  melted  charge  was 
allowed  to  cool  slowly,  undisturbed,  no  return  to  the  crystalline  state 
occurred.  It  merely  thickened  gradually  into  a  transparent  glass 
without  releasing  the  "latent"  heat  which  it  had  taken  on  in  melting 
(fig.  7,  6).  If  it  was  subjected  to  the  jarring  produced  by  the  electric 
hammer  on  the  furnace  wall,  it  cooled  down  a  few  degrees  below  the 
melting  point  and  then  began  to  crystallize,  the  heat  of  fusion  was  set 
free,  and  a  rise  in  temperature  immediately  appeared,  represented  by  a 
hump  upon  the  cooling  curve,  as  shown  in  the  figure  (fig.  5, b,  c,  d) .  Up 
to  this  point  the  phenomenon  differs  but  little  from  the  usual  behavior 
of  liquids  which  undercool  in  solidifying.  We  next  varied  the  experi- 
ment by  first  cooling  quietly  to  about  100°  below  the  melting  point 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


7000 

(775°) 


6500. 


and  then  introducing  a  few  crystal  fragments  or  starting  thfe  pound- 
ing. Crystallization  and  release  of  the  latent  heat  followed  at  once. 
In  fact  over  a  range  of  some  250°  immediately  below  the  melting  point 
it  proved  to  be  within  our  power  to  precipitate  the  crystallization  of 
the  undercooled  mass  entirely 
at  will.  It  was  even  possible 
to  cool  the  melted  charge 
quietly  down  to  the  tempera- 
ture of  the  room  and  remove 
it  from  the  furnace  as  a  clear 
glass,  then,  on  a  subsequent 
day,  to  reheat  to  some  point  in 
this  sensitive  zone  and  pound 
judiciously,  when  crystalliza- 
tion would  at  once  begin, 
marked  by  the  release  of  the 
latent  heat  of  the  previous 
fusion  as  before  (fig.  6,  a,  b). 
The  accompanying  curves 
show  the  situation  clearly. 
Curves  aaf  and  bb',  fig.  7,  were 
obtained  from  charges  of  crys- 
talline and  vitreous  borax,  re- 
spectively, of  exactly  equal 
weight,  which  were  cooled  and 
reheated  in  the  same  electric 
furnace  under  like  conditions. 
The  radiation  from  the  furnace 
for  like  temperature  conditions 
is  practically  the  same,  so 
that  the  more  rapid  rate  of 
cooling  and  of  reheating  in  the 
crystalline  charge  indicates  a 
much  smaller  specific  heat  than 
for  the  vitreous  form. 

From  the  point  of  view  of  the 
usual  definition  of  the  solidify- 
ing point  of  a  substance,  a  dirfi- 


6000 
(680*) 


8 

u 

E 

c  5500 


Q. 

E 

9 
H 

5000 


4500 


4000 
(490°) 


\ 


Time  -  I  division  =»  10  minutes 


FIG.  6. — Curves  showing  the  release  of  the 
heat  of  fusion  at  widely  different  tem- 
peratures. 


culty  confronts  us  here:  (i)  We  were  able  to  vary  the  beginning  of 
solidification  (crystallization)  at  will  over  a  range  of  250°,  and  (2)  the 
temperature  to  which  the  charge  rose  after  the  undercooled  liquid 
had  begun  to  crystallize  did  not  reach  the  melting  point,  although 


BORAX. 


33 


once  crystallization  was  induced  only  10°  below  it  in  a  furnace  of 

constant    temperature.     The 
rapidity  with  which  the  crys- 
tallization and  the  accompany- 
ing release  of  the  latent  heat  go 
on  depends  in  part  upon  the 
rate  of  cooling  and  the  char- 
acter of  the  disturbance  which 
has  been  applied,  i.  e.,  upon 
accidental   rather  than  char- 
acteristic conditions.     It  thus 
happens  that   the  amount  of 
the  heat  of  fusion  and  its  slow 
rate  of  liberation  in  the  case  of 
liquids  which  can  be  greatly 
undercooled  and  become  very 
viscous   may   be    such   as  to 
deprive  it  of  its  usual  signifi- 
cance as  defining  a  solidifying 
point.     It  is,  of  course,  a  con- 
sequence of  the  phase  rule  that 
the  solidifying  temperature  of 
an  undercooled  liquid  is  estab- 
lished, it  only  equilibrium  be- 
tween  solid   and   liquid   (and 
vapor)  is  reached  before  com- 
plete solidification  is  accom- 
plished, but  equilibrium  is  not 
necessarily  attained  during  so- 
lidification, and   the  devices 
usually  employed  (sowing  with 
crystals,  agitating)  are  often 
totally  inadequate  to  effect  it. 
The    temperature  to  which  a 
crystallizing  liquid  rises  after 
undercooling  is  not  necessarily 
constant  or  in  any  way  related 
to   the    melting  point  and  is, 
therefore,  not,  in  general,  enti- 
tled to  be  regarded  as  a  physi- 
cal constant. 
We  then  endeavored  to  ascertain  whether  the  unstable  domain  had 

a  lower  limit  also.     For  this  purpose  we  mixed  a  quantity  of  the 


20 


40  60  80 

Time  (minutes) 


100 


FIG.  7. — Curves  showing  difference  in  spe- 
cific heat  between  crystalline  (aa')  and 
vitreous  (&&')  borax  under  like  condi- 
tions of  cooling  and  reheating. 


34          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

crystals  with  the  glass  and  powdered  them  together  to  about  the  fine- 
ness represented  by  a  i5o-mesh  sieve  and  heated  them  very  slowly. 
In  this  condition  the  glass  proved  to  be  very  unstable  and  crystallized 
readily  with  a  rapid  release  of  its  latent  heat  at  about  490°.  Very 
slow  heating  (10  minutes  per  i  degree)  gave  a  temperature  a  few  de- 
grees lower,  but  such  variations  as  could  be  applied  within  the  period 
of  a  working  day  did  not  suffice,  under  the  most  favorable  conditions, 
to  change  this  temperature  materially.  The  first  evidence  of  molec- 
ular mobility  in  borax  glass,  shown  in  the  sticking  together  of  the 
finest  particles  (sintering),  and  the  first  traces  of  crystallization  and 
release  of  latent  heat,  appeared  consistently  at  about  490°  to  500°. 
Still  a  third  phenomenon  attracted  our  attention  to  this  temperature. 
On  every  occasion  when  borax  glass  was  heated  rapidly,  either  pow- 
dered or  in  the  solid  block,  a  slight  but  persistent  absorption  of  heat 
appeared  in  this  same  region  and  continued  over  some  20°,  after  which 
the  original  rate  of  heating  returned.  We  were  entirely  unable  to 
explain  an  absorption  of  heat  in  an  amorphous  substance  under  these 
conditions  except  by  assuming  an  actual  change  of  state  to  exist 
between  amorphous  glass  and  its  melt,  in  which  case  the  absorbed 
heat  would  reappear  somewhere  upon  the  corresponding  cooling  curve, 
which  it  failed  to  do.  We  then  reasoned  that  any  assumed  change 
in  the  molecular  structure  which  would  account  for  an  absorption  of 
heat  would  also  be  likely  to  cause  an  interruption  in  the  continuity 
of  the  curve  of  electrical  conductivity,  and  the  relative  conductivity 
was  determined  throughout  this  region,  but  no  such  interruption 
appeared. 

Finally  the  matter  was  abandoned.  The  evidence  did  not  appear 
sufficient  to  establish  any  discontinuity  in  the  cooling  curve  of  the 
glass,  so  long  as  no  crystallization  took  place. 

When  these  relations  had  been  clearly  established,  we  turned  again 
to  the  feldspars. 

It  became  clear  very  early  in  the  investigation  that  only  artificially 
prepared  and  chemically  pure  specimens  would  be  adequate  for  our 
purpose.  Each  of  the  end  members  of  the  series,  anorthite  and  albite, 
as  found  in  nature,  is  always  intermixed  with  some  quantity  of  the 
other,  while  the  intermediate  members  generally  contain  iron  and 
potash,  and  all  are  liable  to  inclusions. 

There  was  nothing  new  in  this  plan.  Fouque  and  Levy*  had 
demonstrated  the  possibility  of  making  pure  feldspars  by  chemical 
synthesis  and  had  studied  their  optical  properties  some  years  ago.  We 
undertook  to  prepare  much  larger  quantities  than  they  (200  grams) 


Synthese  des  Mineraux  et  des  Roches. 


ARTIFICIAL  FELDSPARS. 


35 


and  to  make  a  careful  study  of  their  heating  and  cooling  curves  under 
atmospheric  pressure — the  conditions  under  which  anorthite  and  the 
plagioclases  crystallize,  the  relations  between  the  amorphous  and  crys- 
talline forms,  the  sintering  of  crystalline  and  vitreous  powders,  in 
short,  their  entire  thermal  behavior,  as  we  had  done  with  the  borax. 
At  the  same  time  it  was  our  purpose  to  make  careful  determina- 
tions of  the  specific  gravities  of  both  the  vitreous  and  the  crystalline 
products,  analyses  of  such  portions  as  might  be  of  special  interest, 
and  also  to  prepare  microscopic  sections  wherever  they  were  likely 
to  throw  light  on  the  relations  involved.  The  latter,  after  preliminary 
examination,  were  very  thoroughly  studied  by  Prof.  J.  P.  Iddings  of 
the  University  of  Chicago,  whose  large  petrographic  experience  with 
mineral  crystallites  makes  his  judgment  of  very  exceptional  value. 
His  analyses  (see  Part  II)  of  the  slides  form  an  important  part  of  this 
discussion.  We  are  also  indebted  to  Mr.  W.  L/indgren  of  the  United 
States  Geological  Survey  for  valuable  assistance  in  the  microscopical 
study  of  our  products. 

ANALYSES  OF  ARTIFICIAL  FELDSPARS. 


An.                       AbiAns. 

ADlAn2. 

AbaAn^ 

Found. 

Calcu- 
lated. 

Found. 

Calcu- 
lated. 

Found. 

Calcu- 
lated. 

Found 

Calcu- 
lated. 

SiO, 

43-33 
36.21 
.29 
20.06 
.  ii 

43.28 
36.63 

20.09 

47.10 
34-23 
•15 
17.00 

1.74 

47.18 
34-oo 

16.93 
1.87 

51.06 
3I-50 
.  22 
I3-65 

3-68 

51-30 
31.21 

'ia'es 

3-79 

60.  01 

24-95 
.29 
7.09 
7-79 

59.81 

25-47 

6^98 

7-73 

AloO, 

Fe.,O, 

CaO  

Na3O  

100.00 



100.22 

IOO.  II 



100.13 

The  constituents  used  in  our  syntheses  were  precipitated  calcium 
carbonate,  anhydrous  sodium  carbonate,  powdered  quartz  (selected 
crystals),  and  alumina  prepared  by  the  decomposition  of  ammonium 
alum.  None  of  these  contained  more  than  traces  of  impurities,  if  we 
except  the  quartz,  in  which  0.25  per  cent  of  residue,  chiefly  oxide  of 
iron,  was  found  after  treatment  with  hydrofluoric  and  sulphuric  acids. 
All  but  the  calcium  carbonate  were  carefully  calcined  and  cooled  in  a 
desiccator  before  weighing.  To  obtain  a  homogeneous  product,  the 
weighed  constituents  were  mixed  as  thoroughly  as  possible  mechani- 
cally and  heated  in  large  covered  platinum  crucibles  (100  cc.  capacity) 
in  a  Fletcher  gas  furnace.*  After  some  hours'  heating,  during  which 
the  temperature  usually  reached  1500°  or  more,  the  product  was 
removed  from  the  furnace,  cracked  out  of  the  crucibles,  powdered, 


*  Buffalo  Dental  Company,  No.  41  A.     A  Fletcher  furnace  of  this  type,  with 
ordinary  city  gas  pressure  and  a  small  blast  motor,  will  melt  all  of  the  feldspars. 


36          ISOMORPHISM   AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

passed  through  a  "loo-mesh"  sieve,  and  then  melted  again.  This 
process  probably  gives  a  fairly  homogeneous  mixture,  though  a  third 
fusion  in  the  resistance  furnace  was  generally  made  before  determining 
the  constants. 

We  prepared  in  this  way  albite  (Ab),  anorthite  (An),  and  the  follow- 
ing mixtures  of  the  two:  AbiAn5,  AbiAn2,  AbiAni,  Ab2Ani,  AbjAn^ 
Ab4Ani.  All  of  these  were  obtained  in  wholly  or  partially  crystalline 
form,  by  crystallization  from  the  melt,  except  albite.  The  syntheses 
were  controlled  by  analyses  of  a  number  of  the  products,  the  results 
of  which  are  shown  in  the  table  on  p.  35. 

ANORTHITE  (PLATE  I). 

Of  the  whole  series  of  feldspars,  anorthite  is  in  many  respects 
the  simplest  to  deal  with.  It  is  of  relatively  low  viscosity  when 
melted,  and  crystallizes  easily,  very  rapidly,  and  always  in  large,  well- 
developed  crystals.  A  loo-gram  charge  crystallized  completely  in  ten 
minutes.  Sudden  chilling  gave  a  beautiful  clear  glass  entirely  free 
from  bubbles,  somewhat  slower  cooling  usually  resulted  in  a  partial 
crystallization  from  few  nuclei,  the  crystals  always  being  large.  In 
appearance  it  resembles  the  natural  mineral  in  every  respect.  Its 
hardness  is  also  equal  to  that  of  natural  anorthite.  Thin  sections 
show  good  cleavage,  and  twinning  according  to  the  albite  law  is  fre- 
quent. The  extinction  and  other  microscopic  characteristics  are  as 
well  marked  as  in  natural  specimens. 

The  heating  curve  of  crystalline  anorthite  is  perfectly  smooth  except 
for  the  single  break  which  marks  the  melting  point.  No  trace  of  a 
second  crystalline  form  (Umwandlung)  appeared  in  this  or  any  other 
of  the  feldspars  within  the  temperature  range  of  the  observations 
(300°  to  1600°).  Some  undercooling  always  occurs  in  solidification 
even  if  the  rate  of  cooling  is  slow,  but  it  is  less,  under  like  conditions, 
with  anorthite  than  with  any  other  member  of  the  series.  The  heat- 
ing curve  of  the  glass  shows  a  strong  evolution  of  heat  which  may 
occur  as  low  as  700°,  when  crystallization  takes  place.  The  melting 
point  of  crystalline  anorthite  was  determined  by  three  different 
thermo-elements  upon  two  different  mineral  preparations.  It  will  be 
seen  from  the  table  on  p.  37  that  the  determinations  agree  remarkably 
well .  This  is  of  considerable  significance  with  reference  to  the  me tho  d 
of  temperature  measurement  employed.  It  will  be  remembered  that 
the  established  temperature  scale  ends  at  1 1 50°  and  that  temperatures 
beyond  that  point  are  extrapolated  with  the  help  of  some  trust- 
worthy phenomenon  which  varies  with  the  temperature.  We  chose 
for  this  purpose  the  thermo-electric  force  developed  between  pure 


ANORTHITE. 


37 


platinum  and  platinum  alloyed  with  10  per  cent  of  rhodium.  Now 
the  constants  of  such  thermo-elements  will  usually  differ  among 
themselves  and  require  to  be  determined  for  each  element  by  calibra- 
tion with  the  gas  thermometer  or  with  the  melting  points  of  the 
metals.  It  therefore  offers  an  excellent  test  of  the  value  of  the  extra- 
polation if  some  sharp  melting  point  can  be  found  in  the  extrapolated 
range  to  serve  as  a  point  of  reference.  The  melting  point  of  crystal- 
line anorthite  serves  this  purpose  exceedingly  well,  and  separate 
determinations  of  it  with  three  separate  thermo-electric  systems, 
gave  identical  values  within  the  limits  of  error  of  observation.  Our 

ANORTHITE. 
FIRST  PREPARATION. 


Date. 

Element. 

Electromotive 
force    in  MV. 

Tempera- 
ture. 

Remarks. 

Oct.  7,  1903 

A 

15,939 

1534° 

Solid  charge,  open  crucible 

Do. 

A 

15,914 

I532 

Do. 

Oct.  10,  1903 

A 

15,878 

1530 

Covered  crucible. 

Do. 

No.  3 

16,074 

1533 

Do. 

Do. 

No.  3 

16,058 

1532 

Do. 

Do. 

No.  3 

16,068 

1532 

Do. 

Do. 

No.   2 

16,095 

1532 

Do. 

Mean   1532° 

SECOND  PREPARATION. 

Jan.  1  6,  1904 

A 

i  s,86o 

1532°      |  Covered  crucible. 

Do. 

A 

15,864 

1532                     Do. 

Jan.  20,1904 

No.  3 

15,9^0 

1533                     Bo. 

Do. 

No.  2 

16,102 

1532 

Do. 

Do. 

No.  2 

16,092 

1532 

Do. 

Mch.  31,  1904 

No.  3 

15,932 

i53i 

First  and  second  prepara- 

tions together. 

Mean  1532° 

Melting  temperature,  1532°. 

confidence  that  the  extrapolation  for  these  375°  is  reasonably  correct 
would,  therefore,  appear  to  be  justified.  Until  the  gas  scale  can  be 
extended  over  this  range,  the  melting  point  of  pure  anorthite  (1532°) 
determined  in  this  way  will  serve  as  a  useful  point  in  thermometry. 

AB,AN5  (PLATES  II,  III,   IV,  V). 

This  feldspar  decidedly  resembles  anorthite  in  its  relatively  low 
viscosity,  the  readiness  with  which  it  crystallizes,  the  well-marked 
break  in  the  heating  curve  at  the  melting  point,  and  in  its  tendency  to 
form  comparatively  large  crystals.  In  general,  we  may  say  that  all 
these  characteristics  are  somewhat  less  marked  than  in  anorthite. 
Our  determinations  of  the  melting  temperature  follow. 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OP  FELDSPARS. 


FIRST    PREPARATION. 


Date. 

Element. 

Electromotive       Tempera- 
force  in  MV.    j         ture. 

Remarks. 

Dec.     9,   1903 
Dec.   n,   1903 
Do. 
Dec.    12,    1903 
Do. 
Do. 
Do. 

A 
A 

No.  3 
No.  3 
No.  3 
No.  3 
A 

15,501 
15,363 
15,507 
15,599 
15,594 
15,604 
I5,5i8 

I5040 
1493 
1498 

1505 
1505 
1506 

1505 

Slow  heating. 
Rapid  heating. 
Do. 
Do. 
Do. 
Slow  heating. 
Do. 

Mean   1502° 

SECOND  PREPARATION. 

Apr.    9,  1904 
Do. 

No.  3 
No.  2 

15,520 
15,637 

1499° 
1497 

Slow  heating. 
Do. 

Mean  1498° 

Melting  temperature,  1500°. 

In  one  instance,  while  cooling  the  molten  mass  at  a  rapid  rate,  we 
obtained  a  result  which  has  a  most  important 
bearing  on  the  relation  of  the  feldspars  to  one 
another,  which  will  be  referred  to  again  in  the 
concluding  discussion  of  the  experimental  data. 
When  the  charge  had  cooled,  it  was  found  to 
consist  of  a  compact  mass  of  rather  large  crystals, 
radial  in  structure,  at  the  bottom  of  the  crucible 
(fig.  8),  and  a  beautiful,  transparent  glass  above. 
It  was  easy  to  separate  the  crystalline  portion 
from  the  glass  and  to  analyze  the  two  separately. 
The  composition  of  the  two  portions  is  practically 
identical,  save  for  a  slightly  higher  percentage  of 
-iron  in  the  crystals.  (A  small  quantity  of  iron 
was  contained  in  the  quartz  used  in  preparing 
the  feldspars.)  In  harmony  with  this  latter 
circumstance  the  color  of  the  crystals  was  a 

decided  amethyst  brown,  while  the  glass  was  but  slightly  tinted. 

The  analyses  follow: 


. 

Glass 
residue. 

Crystalline 
cake. 

Si02  
ALO. 

47.46 
33.  56 

47-34 

T.T.  .  SI 

Fe,0s  
CaO  
NagO  

.29 
16.99 
l,87 

•47 
16.84 
1.89 

100.17 

100.05 

INTERMEDIATE   FELDSPARS. 


39 


It  is  at  once  clear  from  these  determinations  that  the  solid  phase 
has  the  same  composition  as  the  liquid  phase,  so  far  as  it  is  within  the 
power  of  chemical  analysis  to  establish  it. 

AB,ANS  (PLATES  VI,  VII,  VIII,  IX,  X,  XI). 

In  this  feldspar  we  observe  the  same  characteristics  as  in  the  two 
preceding,  but  they  are  still  less  sharply  marked.  The  viscosity  is 
greater,  both  solidification  and  melting  take  place  more  slowly,  and 
the  undercooling  is  so  persistent  that  the  furnace  must  be  cooled 
slowly  or  the  charge  will  come  out  wholly  or  partly  vitreous. 


FIRST  PREPARATION. 


Date. 

Element. 

Electromotive 
force  in  MV. 

Tempera- 
ture. 

Remarks. 

Oct.   1  6,  1903 

A 

14,895 

1459° 

Rapid  heating. 

Do. 

No.  3 

15,14? 

1460 

Slow  heating. 

Oct.  21,  1903 

No.  3 

15,101 

1457 

Rapid  heating. 

Do. 

No.  3 

15,220 

1466 

Extremely  slow  heating. 

Oct.   22,    1903 

No.  3 

15,204 

1465 

Rapid  heating. 

Do. 

No.  3 

15,160 

1462 

Dec.  15,  1903 

No.  3 

15,116 

1467 

Powdered  charge,  open 

crucible. 

Do. 

No.  3 

15,103 

1466 

Powdered  charge,  slower. 

Dec.  1  6,  1903 

No.  3 

15,109 

1467 

Solid  cake,  covered. 

Do. 

No.  3 

15,044 

1462 

Very  fast. 

Do. 

No.  3 

15,040 

1462 

Same,  slower. 

Do. 

A 

15,035 

1467 

Mean  1463° 

SECOND  PREPARATION. 

Feb.  19,  1904 

A 

14,945 

1460° 

Covered,  slow. 

Feb.  20,  1904 

No.  3 

15,096 

1466 

Covered,  faster. 

Feb.  25,  1904 

No.  2 

15,239 

1467 

Fast. 

Mean  1464° 

Melting  temperature,  1463°. 

Here  again  we  made  an  attempt  to  discover  a  possible  difference  in 
composition  in  the  first  portions  to  crystallize  out  of  the  melt,  this 
time  by  optical  means.  We  first  cooled  the  charge  so  rapidly  that 
only  a  relatively  small  portion  crystallized  out  in  fine,  reddish-brown 
spherulites  at  the  surface  and  near  the  wall  of  the  crucible.  Without 
disturbing  these,  the  crucible  was  then  replaced  in  the  furnace  and 
slowly  reheated  (about  five  hours)  until  the  remaining  vitreous  mate- 
rial had  also  become  completely  crystallized.  Upon  removing  from 
the  furnace,  the  charge  presented  a  singular  appearance.  The  red- 
dish-brown stars  remained  undisturbed,  while  the  later  crystals  were 
perfectly  white.  But  though  so  different  in  appearance,  the  micro- 


40          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

scopic  examination  of  slides  cut  from  the  different  portions  showed 
the  two  to  be  optically  identical. 

We  have  here  another  instance  of  the  tendency  of  the  iron  to  con- 
centrate in  the  crystals  which  first  form,  a  tendency  which  was  often 
noticed  throughout  our  work.*  It  also  appeared  to  matter  little 
whether  the  first  crystals  formed  at  the  surface  or  at  the  bottom  of  the 
charge.  This  phenomenon  may  have  significance  in  ore  deposition, 
but  we  have  not  thus  far  been  able  to  give  it  adequate  attention. 

ABjAN!  (PLATES  XII,  XIII). 

With  this  member  of  the  feldspar  group  a  difficulty  in  effecting 
crystallization  in  the  molten  mass  becomes  noticeable.  Undercooling 
will  continue  until  the  vitreous  melt  becomes  rigid,  unless  the  cooling 
is  slow  or  some  special  effort  in  the  way  of  mechanical  disturbance  or 
the  introduction  of  nuclei  is  applied.  Furthermore,  when  once  pre- 
cipitated, crystal  formation  goes  on  slowly,  even  when  the  charge  is 
finely  powdered,  and  the  crystals  are  always  small.  Of  the  feldspars 
at  least  it  is  possible  to  say  that  the  size  of  individual  crystals  varied 
chiefly  with  the  viscosity;  the  thinner,  calcic  feldspars  always  gave 
large  individuals,  while  AbiAni,  Ab2Ani,  Ab3Ani  and  Ab4Ant  crystal- 
lized in  closely  interwoven,  increasingly  smaller  fibers,  which  gave 
much  difficulty  in  microscopic  study.  In  comparison  with  this  appar- 
ent effect  of  the  viscosity,  the  rate  of  cooling  was  altogether  insignifi- 
cant in  determining  the  size  of  individual  crystals. 

Several  days  were  required  to  complete  the  crystallization  of  100 
grams  of  AbiAni  under  the  most  favorable  conditions  which  we  were 
able  to  bring  to  bear  upon  it.  The  melting  temperature  of  the  crys- 
talline feldspar  was  still  fairly  well  marked,  however,  and  crystalliza- 
tion began  in  the  powdered  vitreous  material  as  low  as  700°. 

The  melting  point  of  this  feldspar  is : 


Date. 

Element. 

Electromotive 
force  in  MV. 

Tempera- 
ture. 

Remarks. 

Feb.  9,  1904 

A 

14,402 

1416° 

Covered  charge,  heating 

I 

rapid. 

Do. 

A 

14,400                 1416 

Feb.  10,  1904 

No.  3 

14,529 

1421 

Very  rapid. 

Feb.  12,  1904 

No.  2 

14,572 

1415 

Feb.  27,  1904 

No.  2 

14,709 

1426 

Very  small  charge. 

Mean    1419° 

Melting  temperature,  1419° 


*See  also  J.  P  Iddings,  Bull.  Phil.  Soc.  Wash.,  Vol.  XI,  p.  97,  1888-1891 


INTERMEDIATE  FELDSPARS. 


To  effect  the  complete  crystallization  of  this  substance,  it  is  best  to 
reduce  it  to  a  fine  powder  and  heat  very  slowly,  holding  the  temper- 
ature for  many  days  at  100°  to  200°  below  the  melting  point.  When 
thoroughly  crystallized,  it  has  a  melting  temperature  which  is  deter- 
minable  with  reasonable  certainty,  but  neither  this  nor  any  of  its 
thermal  phenomena  approach  the  more  calcic  feldspars  in  sharpness. 
For  this  reason  a  considerably  greater  variation  will  be  noticed  in  the 
melting  points  tabulated  below: 


FIRST  PREPARATION. 


Date. 

Element. 

Electromotive 
force  in  MV. 

Tempera- 
ature. 

Remarks. 

Dec.  10,  1903 
Dec.  15,  1903 
Dec.  1  6,  1903 
Do. 
Jan.  1  8,  1904 
Feb.  29,  1904 
Do. 

A 
A 
A 
A 
No.    } 
No.  3 
No.  3 

13,726 
13,887 

13,969 
13,728 

13,967 
13,812 

13,854 

I362C 

1374 
1381 
1362 
1376 

1363 
1366 

Very  rapid  heating. 
Poor. 
Covered. 
Do. 
Do. 

Mean    1369° 

SECOND  PREPARATION. 

Feb.  5,  1904 

No.  2 

13,990 

1369° 

Covered. 

THIRD  PREPARATION. 

Mch.  25,  1904 
Mch.  29,  1904 
Apr.  5,  1904 

No.  3 
No.  2 
No.  3 

13,752 
13,995 
13,756 

1358° 
1370 
1358 

Mean   1362° 

Melting  temperature,  1367°. 

From  here  on  to  the  albite  end  of  the  series,  viscosity  becomes  very 
troublesome  in  restraining  crystallization.  The  breaks  which  mark 
the  melting  temperature  on  the  heating  curve  of  AbjAni  are  so  slight 
as  to  make  the  determination  difficult  and  somewhat  uncertain.  It  is 
not  that  temperature  measurement  is  less  accurate  here  than  else- 
where, for  these  temperatures  are  more  accessible  than  the  melting 
point  of  anorthite  to  which  reference  has  been  made  in  this  connection. 
These  ultra-viscous  materials  do  not  melt  at  a  constant  temperature 
but  over  a  considerable  range  of  temperature,  as  we  shall  undertake 
to  show  in  some  detail,  with  illustrations  from  photographs,  in  the 
discussion  of  albite.  A  glance  at  a  series  of  curves  (fig.  9)  plotted 
from  our  observations  upon  metallic  silver  and  the  feldspars  An, 
AbiAn5,  AbiAn2  and  AbiAni,  in  such  a  way  as  to  bring  their  melting 
points  together,  will  show  clearly  the  nature  of  this  difficulty.  The 
melting  point  of  the  metal  is  sharp,  but  with  anorthite  a  change  in  the 


42          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


character  of  the  phenomenon  is  noticeable.  Its  poor  conductivity  for 
heat  and  its  viscosity,  which,  though  small  compared  with  the  other 
feldspars,  are  very  great  compared  with  silver,  have  rounded  off  the 
corners  until  a  really  constant  temperature  for  a  period  of  a  minute  or 
more  during  the  melting  is  nowhere  to  be  found.  The  nearest  ap- 


Time  -  I  division  =  5  minutes 
FIG.  9. — Melting  point  curves  of  various  feldspars  compared  with  silver. 

proach  to  a  melting  point  is  where  the  rise  in  temperature  is  slowest, 
and  this  will  occur  when  the  portion  nearest  to  the  thermo-element 
(see  fig.  3)  melts. 

A  series  of  melting-point  curves  containing  a  typical  one  for  each  of 
the  observed  feldspars  is  reproduced  on  page  43  exactly  as  observed : 


INTERMEDIATE  FELDSPARS. 


43 


TIME  CURVES. 
(In  microvolts  as  observed). 


An. 

Ab,An,,. 

Ab1Ana. 

AblAni. 

Ab2Ani 

Ab3Ani. 

MF. 

AF. 

MF. 

AF. 

MF. 

AF. 

MF. 

AF. 

MF. 

AF. 

MF. 

AF. 

15050 
15206 
I534I 
I544I 
15526 

15594 
15650 
15697 

156 
135 
100 

85 

68 
56 

47 

13530 
13830 
14130 
14370 
14560 

14838 
14942 

300 
300 
240 
190 

153 
125 
104 

Qfi 

11700 
I22IO 

12655 
12960 
13236 
I344I 
13595 
13722 

510 

445 
305 
276 
205 

154 
127 

I  I  O 

13400 
13489 
13573 
13647 
I37I5 
13778 
13836 
13891 

89 
84 
74 
68 

63 
58 
55 

12480 

12533 
12590 
12648 
12701 
12752 
12797 
12840 

53 
57 
58 
53 

45 
43 

12754 
12793 
12834 

12877 
12916 

12954 
12989 
13022 

39 

43 
39 
38 
35 
33 

15738 
15773 
15802 
15829 
15853 
15875 
15891 
15906 
15920 
15933 
15945 
15956 
15965 
15974 
15983 
15992 
15998 
16004 

41 
35 
29 
27 

24 

22 

16 

15 
14 
13 
12 
I  I 

9 
9 
9 
9 
6 
6 

15028 
I5IOI 
15164 
15218 
15264 
15303 
15339 
I537I 
15398 
15423 
15447 
15468 
15488 
15504 
I552I 
15537 
15552 
15567 

OO 

73 
63 
54 
46 
39 
36 
32 
27 
25 
24 

21 
2O 

16 

17 
16 

15 
15 

13832 
13932 
14022 
14107 
14186 
14256 
H323 
14393 
14456 
I45H 
H57I 
14620 
14670 
14714 
14759 
14797 
14839 
14877 

IOO 

90 
85 
79 
70 

67 
70 
63 
58 
57 
49 
50 
44 
45 
38 
42 
38 

i  r 

13942 
I399I 
14038 
14083 
HI25 
14166 
14206 

14245 
14284 

14323 
14363 
14402 
14444 
14488 
14538 
14605 
14676 

49 
47 
45 
42 

40 
39 
39 
39 
40 

39 
42 
44 
50 
67 

12881 
12917 
12952 
12987 
13020 

13053 
13088 
I3I2I 

I3I54 
13184 

I32I5 
13248 
13283 
I33I8 
13355 
13388 
13421 
I345I 

36 
35 
35 
33 
33 
35 
33 
33 
30 

33 
35 
35 
37 
33 
33 
30 

13053 
13082 
13109 

I3I34 
13160 
13184 
13207 
13229 
13250 
13270 
1328? 
13306 
I332I 
13337 
13354 
13373 
13393 
I34I5 

29 
27 
25 
26 

24 
23 

22 
21 
20 

18 
18 

15 
16 

17 
19 

20 

22 

O  A 

I60I2 
16020 

8 

I558I 
15594 

13 

14912 
14947 

35 
35 

T  C 

13483 
I35I6 

1 

13439 
13466 

24 

27 
0*7 

16028 

15608 

I4 

14982 

35 

13547 

on 

13493 

•*/ 

0*7 

16033 

5 

15622 

J4 

I50I3 

^  •} 

13576 

^y 
26 

13520 

•*/ 

28 

16040 
16048 

8 

Q 

15637 
15654 

17 

To 

15046 
15082 

33 
36 

13602 
13627 

25 
2  j 

13548 
13576 

28 

16056 

o 

15672 

1  o 

15118 

3 

13648 

TQ 

16064 

15695 

23 

ift 

15160 

42 

13664 

I  o 

16071 

7 

15733 

3s 

I52II 

51 

f\  C 

13675 

I  I 

sy  T 

16078 

7 

15796 

3 

15306 

95 

13696 

-  1 

'28 

16087 

I54J9 

13724 

16099 

I  2 
j  j 

13758 

36 

l6lIO 

13794 

~ 

16122 

I  2 

13833 

39 

16134 

1  2 

13873 

4° 

16149 

* 

I39I4 

40 

16163 

T  ft 

13954 

•?** 

16181 

lo 

16 

13998 

44 

16197 

16 

14050 

16213 

IQ 

16232 

*  V 

The  numbers  represent  the  electromotive  force  of  the  thermo- 
elements at  intervals  of  one  minute,  together  with  a  column  of  dif- 
ferences at  the  right  of  each  record.  The  E.  M.  F.  will  be  seen  to 
approach  a  minimum  as  melting  progresses  and  to  increase  again 
when  it  is  complete.  This  minimum  rise  in  the  temperature,  of  course, 
indicates  the  maximum  absorption  of  heat.  For  purposes  of  rough 
orientation  10  MV  may  be  considered  equivalent  to  one  degree. 


44         ISOMORPHISM   AND    THERMAL  PROPERTIES  OF    FELDSPARS. 


There  is  no  circulation  in  these  viscous  melts  and  nothing  to  assist 
in  distributing  the  heat  uniformly.  The  melting  point  is,  therefore, 
not  marked  by  a  constant  temperature  but  by  the  point  of  greatest 
inclination  of  the  tangent  to  the  curve,  with  a  limit  of  error  which 
increases  with  increasing  viscosity.  With  Ab3Ani  it  was  barely  dis- 
cernible, and  with  Ab^Ani  all  trace  of  the  heat  of  fusion  was  lost.* 
Slow  heating  or  rapid  heating  merely  acts  to  change  the  general  incli- 
nation of  the  curve  but  not  to  emphasize  the  absorption  of  heat. 

By  way  of  conveying  a  concrete  impression  it  may  be  added  that 
Ab3Ani  just  above  its  melting  temperature  resists  the  introduction  of 
a  stout  platinum  wire  (1.5  mm.  diameter)  unless  the  cold  wire  is  thrust 
in  very  quickly  and  vigorously.  If  the  wire  is  first  allowed  to  become 
hot  in  the  furnace,  it  will  give  way  itself  instead.  No  acceleration  of 
the  melting  process  tending  to  sharpen  the  break  in  the  curve  appears 
to  be  possible  without  the  introduction  of  new  substances  or  new  con- 
ditions (water  vapor  under  pressure  for  example)  which  would  take 
the  experiment  outside  the  definition  of  a  "dry  melt."  We  have 
undertaken  some  preliminary  experiments  in  these  directions,  but 
they  belong  to  another  phase  of  the  subject. 

A  number  of  efforts  were  made  to  locate  the  melting  temperature  of 
Ab3Ani,  which  are  given  in  the  list  below.  Although  two  days  were 
required  to  crystallize  each  charge  of  the  material  sufficiently  for  a 
determination,  the  recorded  numbers  possess  but  little  significance,  as 
will  be  clear  from  the  foregoing. 

AB8ANi   (PLATES  XIV,  XV,  XVI). 
FIRST  PREPARATION. 


Date. 

Element. 

Electromotive 
force  in  MV. 

Tempera- 
ture. 

Nov.  23,  1903 
Nov.  25,  1903 
Nov.  28,  1903 
Dec.  26,  1903 
Jan.  14,  1904 

A 

1 

A 

No.  3 

I3,4I5 
13,698 

13,319 
13,893 

I3250 
1336 
1359 
1328 
1370 

Mean  1344° 

SECOND  PREPARATION. 

Mch.  ii,  1904 
Mch.  14,  1904 

A 

No,  3 

13,218 
13,469 

1320° 
1335 

Mean  1329° 

Approximate  melting  temperature,  1340' 

*  Only  a  small  portion  of  the  charge  could  be  crystallized.  The  relatively  small 
heat  of  fusion  of  the  crystallized  portion  was,  therefore,  superposed  upon  the 
larger  specific  heat  of  the  glass.  This,  together  with  the  effect  of  the  viscosity, 
destroyed  all  record  of  the  melting. 


45 


ABjAN!.       (PLATE  XVII). 

With  Ab4Ani  a  third  proof  of  the  identity  of  composition  of  the  first 
crystals  to  separate  and  the  vitreous  residue  was  obtained.  The 
optical  identification  of  this  feldspar  is  absolute.  If  we  could  obtain 
crystals  at  all  in  a  melt  of  this  chemical  composition,  therefore,  it 
would  offer  a  crucial  test  of  the  relation  of  the  solid  and  liquid  phases 
in  a  part  of  the  curve  where  no  melting  point  or  specific  gravity  deter- 
mination upon  crystals  was  possible.  After  some  days  of  nearly  con- 
tinuous heating  at  a  temperature  somewhat  below  its  assumed  melting 
point,  a  number  of  crystals  of  Ab4Ani  were  obtained  and  identified. 


From  the  experiments  upon  natural  albite  and  orthoclase,  which 
have  been  described,  and  after  observing  the  effect  of  the  increasing 
viscosity  as  we  approached  the  albite  end  of  the  artificial  plagioclase 
series,  we  had  no  expectation  of  finding  a  melting  point  for  either  in 
the  ordinary  sense.  Nor  did  we  in  fact  succeed  in  locating  a  point  of  any 
real  significance  in  this  connection.  The  various  trials  which  were  made 
were  simply  calculated  to  throw  all  the  light  possible  upon  the  char- 
acter of  the  change  from  (crystalline)  solid  to  liquid  in  such  extremely 
viscous  substances.  The  return  change  or  recrystallization  of  such 
substances  from  the  melt  (solidifying  point)  without  the  introduction 
of  modifying  conditions  has  never  been  accomplished.  The  time 
required  to  do  it  is  certainly  very  great,  probably  much  greater  than 
the  demonstration  is  worth  at  the  present  stage  of  experimentation  in 
this  field. 

Crystalline  albite  has  been  produced  under  exceptional  conditions 
several  times  —  by  Hautefeuille,*  by  heating  a  very  alkaline  alumino- 
silicate  with  sodium  tungstate  for  30  days  at  900°  to  1000°  ;  by  Friedel 
and  Sarasinf,  using  an  atmosphere  of  water-  vapor  under  very  high 
pressure  and  a  moderately  high  temperature  (an  aqueo-igneous  fusion)  ; 
by  J.  LenarcicJ,  at  ordinary  pressure  and  high  temperature  by  crys- 
tallization out  of  a  mixture  of  melted  albite  and  magnetite  (i  part 
magnetite,  2  parts  albite  by  weight),  and  by  others.  It  may  be  noted 
in  passing  that,  entirely  apart  from  the  solution  relations,  the  last- 
mentioned  process  reduces  the  viscosity  to  an  entirely  different  order 
of  magnitude  from  that  of  pure  albite  ;  magnetite  melts  to  form  a  thin 
liquid  almost  of  the  consistency  of  water  and  even  in  i  :  10  solution 
with  albite  forms  a  fairly  mobile  liquid.  We  endeavored  to  repeat 

*  Hautefeuille,  Annales  de  I'^cole  Normale  Superieure,  2d  sen,  9,  p.  363,  1880. 
f  Friedel  &  Sarasin,  Bull.  Min.,  p.  158,  1879;  P-  7*.  1881. 
J  J.  Lenarcic,  Centralblatt  f.  Min.,  23,  p.  705,  1903. 


46          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

portions  of  the  work  of  Hautefeuille  and  Lenarcic,  but  were  obliged 
to  postpone  a  systematic  inquiry  into  the  conditions  of  crystallization, 
which  involved  the  addition  of  other  components  or  extraordinary 
pressures,  until  our  plant  could  be  somewhat  extended. 

Hautefeuille  describes  his  successful  preparation  as  a  "solution"  of 
the  alkaline  alumino-silicate  in  sodium  tungstate,  out  of  which  the 
albite  slowly  crystallizes  after  long  heating,  but  he  remarks  that  the 
crystallization  does  not  take  place  if  the  mixture  is  heated  sufficiently 
to  melt  the  components  of  the  charge  into  a  homogeneous  glass.  In 
that  case  he  obtained  only  a  vitreous  white  enamel.  His  case  does 
not  appear,  therefore,  to  be  one  of  simple  solution,  out  of  which  the 
same  solid  phase  always  reappears  upon  reproducing  given  conditions 
of  temperature  and  concentration.  On  the  contrary,  as  Hautefeuille 
describes  the  experiment,  the  components  of  the  albite  remain  as  inde- 
pendent solid  phases,  which  are  then  assembled  in  some  manner 
through  the  intermediary  action  of  the  melted  tungstate. 

Notwithstanding  the  fact  that  our  interest  was  confined  for  the 
moment  to  the  mere  production  of  a  small  quantity  of  chemically  pure 
crystalline  albite,  we  ventured  to  proceed  along  the  lines  of  Haute- 
feuille's  unsuccessful  trial.  We  first  prepared  a  chemically  pure 
albite  glass,  i.  e.,  we  melted  the  components  into  a  homogeneous  mass 
before  adding  tungstate.  This  glass  was  then  finely  powdered,  thor- 
oughly mixed  with  an  excess  of  powdered  sodium  tungstate,  and 
maintained  continuously  for  8  days  at  1100°.  Upon  removing  from 
the  furnace  at  the  close  of  the  heating,  both  albite  and  tungstate  were 
found  to  have  been  completely  melted  and  to  have  separated  into  two 
distinct  layers  according  to  their  specific  gravities,  the  albite  glass 
being  above,  and  showing  no  trace  of  crystallization.  A  second  charge 
was  then  prepared  with  equal  parts  of  tungstate  and  albite,  powdered 
and  mechanically  mixed  as  before,  and  maintained  at  a  temperature 
of  900°  for  1 7  days.  This  time  we  were  successful.  After  the  sodium 
tungstate  had  been  dissolved  away  with  water,  the  albite  appeared  as 
a  powder  of  about  the  fineness  to  which  it  had  originally  been  pulver- 
ized, except  that  the  fragments  were  now  crystalline  and  apparently 
homogeneous  albite.  In  thin  section,  under  the  microscope,  to  our 
considerable  surprise,  it  appeared  that  the  original  glass  fragments 
were  unchanged  in  form.  The  bounding  surfaces  were  all  conchoidal 
fractures,  as  they  came  from  the  hammer,  and  evidently  had  not  been 
in  solution  with  the  tungstate  at  all.  Its  optical  properties  showed  it 
to  be  undoubted  albite  and  the  specific  gravity  was  2.620. 

The  preparation  of  albite  which  we  had  synthesized  by  heating 
with  an  equal  weight  of  sodium  tungstate  was  first  purified  by  thor- 


47 


ough  washing  with  warm  water,  but  this  was  not  sufficient  to  remove 
all  the  tungstate.  A  determination  of  tungstic  acid  showed  0.62  per 
cent  still  present,  which  is  equivalent  to  0.78  per  cent  of  sodium  tungs- 
tate. After  removing  the  water  by  heating  carefully  to  a  dull  redness, 
the  product  was  submitted  to  a  microscopic  examination,  which 
showed  it  to  be  entirely  crystalline  and  apparently  homogeneous. 
Determinations  of  the  specific  gravity  gave  2.620  (see  table,  p.  58). 
If  this  is  corrected  for  0.78  per  cent  of  sodium  tungstate  of  specific 
gravity  4.2,  we  obtain  2.607. 

A  portion  of  the  preparation  was  then  purified  further  by  fusing 
for  a  few  minutes  with  acid  sodium  sulphate  (Hautefeuille)  at  as  low 
a  temperature  as  practicable,  after  which  the  excess  of  sulphate  was 
extracted  with  water  and  the  product  dried  (the  temperature  was 
raised  to  a  dull  red  heat  to  remove  all  water)  and  analyzed. 


Found. 

Calculated 

SiO2  
Ala63  and  Fe3Os  .  .  . 
Na.>O  

68.74 
19.56 
ii  .  73 

68.68 
19.49 
11.83 

SO, 

02 

WO  3 

16 

IOO.  21 

The  specific  gravity  of  it  was  2.604,  which  may  be  corrected  as 
before  for  the  remaining  trace  of  tungstic  acid  assumed  to  be  in  the 
form  of  the  sodium  salt.  The  value  then  falls  to  2.601. 

A  second  portion  of  the  same  albite  was  purified  by  another  process. 
Instead  of  fusing  with  acid  sodium  sulphate,  the  powdered  sample 
was  first  digested  for  a  short  time  with  dilute  hydrochloric  acid  (i :  i), 
which  set  free  tungstic  acid.  The  excess  of  hydrochloric  acid  was 
removed  with  water,  the  tungstic  acid  with  ammonia,  and  finally  the 
excess  of  reagent  and  the  ammonium  tungstate  by  further  washing 
with  water.  When  dried  at  a  low  red  heat,  the  preparation  had  the 
following  composition : 


Found. 

Calculated. 

SiO,  
Al,6,  
Fe303  
Na.O  
WO-| 

68.91 

I8:?i'f9,3 

n-59 

.  22 

68.68 
19.49 
11.83 

Ho 

I  ^ 

00.08 

48 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


The  specific  gravity  determination  gave  2.615,  which,  when  cor- 
rected for  the  small  quantity  of  sodium  tungstate  becomes  2.612.  If, 
as  is  possible  after  the  above  treatment,  the  tungstic  acid  is  present 
as  the  anhydride,  sp.  gr.  7.1,  the  correction  would  lower  the  value 
to  2.605,  m  excellent  agreement  with  the  other  determinations. 

The  products  of  both  methods  of  purification  were  carefully  scrutin- 
ized by  the  microscope,  but  no  conclusion  could  be  reached  as 
to  which  was  the  purer.  Neither  the  sodium  sulphate  fusion,  nor 
the  digestion  with  acid  and  ammonia  appeared  to  have  changed  the 
particles  in  the  slightest  degree.  Diligent  search  was  made  for  opaque 
or  amorphous  matter  on  the  surface  of  the  grains,  or  any  other  indica- 
tion of  decomposition,  but  none  was  found.  While  the  chemical  anal- 
ysis indicates  a  rather  higher  purity  for  the  first  product,  purified  by 
fusion,  the  differences  are  nearly  within  the  limits  of  error  and,  there- 
fore, hardly  conclusive.  Both  powders  were  ground  finer  than  usual 
for  the  specific  gravity  determinations  to  avoid  errors  introduced  by  a 
spongy  structure. 

Reverting  now  to  Hautefeuille's  directions,  it  is  clear  that  glass  of 
albite  composition  crystallizes  homogeneously  under  substantially  the 
conditions  which  he  obtained,  as  well  or  better  than  the  mechanically 
mixed  component  parts ;  but  the  part  played  by  the  tungstate  requires 
some  further  experimental  study  before  a  conclusion  can  be  reached . 

Except  for  the  specific  gravity,  the  experiments  upon  crystalline 
albite  and  orthoclase  which  follow  were  made  upon  natural  specimens 
from  well-known  localities  (a  fragment  of  the  Mitchell  County  albite  is 
shown  in  plate  XVIII),  for  which  we  are  indebted  to  Dr.  G.  P.  Merrill  of 
the  United  States  National  Museum  and  Dr.  Joseph  Hyde  Pratt,  State 
Mineralogist  of  North  Carolina.  The  specimens  were  selected  with 
great  care,  but  like  all  natural  specimens,  they  contained  other  f  eld  - 
spars  and  inclusions.  The  analyses  follow : 


Albite,  Amelia  Co.,Va. 
Nat.  Mus. 

Albite,  Mitchell  Co.,  N.C. 
(Pratt). 

Orthoclase,  Mitchell  Co.. 
N.  C.,  Nat.  Mus. 

Found. 

Calculated  to 
anhydrous 
composition. 

Found. 

Calculated  to 
anhydrous 
composition. 

Found. 

Calculated  to 
anhydrous 
composition. 

SiO, 

68.22 
19.06 

•15 
.40 
11.47 
.  2O 
.69 

68.71 
19.  2O 

•15 
.40 

"•53 
.  20 

66.03 
20.91 
.18 
2.00 

9-97 
.70 

•59 

66.42 
21.03 
.18 
2.00 
10.03 
.JO 

65-49 
17.98 
.36 
.42 
2.29 
12-95 
•51 

65-83 
18.07 
•36 
.42 
2.30 
13.02 

A130S  
Fe203  
CaO  

Na,O  
K,O 

HgO 

100.  19 

100.38 

IOO.OO 

ALBITK. 


49 


It  will  be  remembered  that  in  the  preliminary  experiments  (p.  28 
et  seq.)  the  heating  curve  of  these  natural  feldspars  did  not  show  an 
absorption  of  heat  which  we  were  able  to  detect ;  our  first  step  was, 
therefore,  to  find  out  what  manner  of  process  it  was  by  which  a  charge 
of  crystalline  albite  or  orthoclase  became  amorphous  without  leaving 
a  thermal  record  behind. 

We  prepared  a  charge  of  albite  glass  from  a  previous  melt  powdered 
to  "  loo-mesh."  In  this  glass  powder  a  small  crystal  fragment  (per- 
haps 2  x  5  x  10  mm.)  from  the  same  original  specimen  and,  therefore, 
of  the  same  chemical  composition,  was  embedded  beside  the  thermo- 


ii. 


FIG.  10. — Albite  crystal  embedded  in  charge  of  powdered  albite  glass. 
FIG.  ii. — Same  after  heating. 


element  as  indicated  in  fig.  10.  This  charge  was  heated  slowly  to 
exactly  1200°,  slowly  cooled  again,  and  several  thin  sections  prepared 
from  the  crystal  fragment  and  its  immediate  neighborhood.  What 
the  microscope  showed  can  best  be  seen  from  the  accompanying 
illustrations  (Plate  XX) — groups  of  crystal  fragments  of  microscopic 
size,  preserving  their  original  orientation  (extinction)  perfectly,  but 
with  narrow  lanes  of  glass  where  cleavage  and  other  cracks  had  been, 
forming  a  perfect  network  without  a  trace  of  disarrangement.  Con- 
siderable melting  had  taken  place  but  no  flow.  Neither  had  the 
charge  as  a  whole  made  any  movement  to  take  the  form  of  the  con- 
taining vessel  after  sintering  together  (fig.  n). 


50         ISOMORPHISM  AND  THERMAL   PROPERTIES  OF  FELDSPARS 

Surmising  that  we  had  accidentally  hit  upon  the  approximate  melt- 
ing temperature,  a  fresh  charge  of  like  material  was  prepared  and  the 
same  experiment  carefully  repeated,  except  that  the  temperature  was 
carried  up  to  1206°  and  maintained  there  for  30  minutes.  Instead 
of  showing  the  melting  to  be  complete,  the  slides  (Plate  XXI)  looked 
precisely  like  the  first,  save  that  the  lanes  of  glass  were  somewhat  wider 
and  the  crystal  fragments  relatively  smaller  than  before.  Further 
trials  under  precisely  the  same  conditions,  with  the  temperature  in- 
creased to  1225°  (Plate  XXII)  and  1250°  (Plate XXIII),  respectively, 
for  like  periods  of  time,  showed  only  more  advanced  stages  in  the  same 
process.  In  the  latter  case  the  remaining  crystal  fragments  were  rela- 
tively very  small  compared  with  the  separating  lanes  of  glass,  but  the 
orientation  of  the  tiny  particles  still  remained  perfectly  undisturbed. 

The  evidence  contained  in  this  series  of  slides  shows  plainly  that  we 
have  here  an  unfamiliar  condition — a  case  of  a  crystalline  compound 
persisting  for  a  long  time  above  its  melting  temperature  for  a  given 
pressure.  Albite  or  orthoclase  glass  sinters  tightly  at  800°.  At  the 
temperature  where  melting  began,  therefore  (below  1200°),  the  charge 
consisted  of  crystal  fragments  of  microscopic  size  embedded  in  a  large 
vitreous  mass  of  the  same  composition  and  known  temperature. 
These  fragments  melted  so  slowly  over  the  50°  included  between  the 
first  slide  and  the  last,  with  the  rate  of  heating  slow  (i°  in  2  minutes) 
and  the  upper  temperature  continued  for  30  minutes,  as  to  leave  con- 
siderable portions  unmelted  at  the  close.  Furthermore,  the  extreme 
viscosity,  of  which  further  evidence  will  be  given  directly,  and  the 
absence  of  any  disturbance  in  the  orientation  of  the  particles  indicat- 
ing flow,  assured  us  that  the  lanes  of  glass  represented  actual  melting 
and  not  an  inflow  of  glass  from  without.  Finally,  the  perfectly  homo- 
geneous character  of  the  glass  and  the  unchanged  appearance  of  the 
crystals  as  heating  progressed  gave  no  hint  of  any  chemical  decom- 
position. 

In  the  hope  of  obtaining  a  point  of  value  for  comparison  with  the 
melting  points  of  the  other  feldspars,  some  time  and  patience  were 
expended  in  trying  to  locate  the  lowest  temperature  at  which  certain 
evidence  of  melting  appeared.  We  did  not  extend  any  single  trial 
beyond  a  single  day,  so  that  our  results  can  not  pretend  to  establish 
the  lowest  point  at  which  albite  melts.  Such  an  effort  with  a  natural 
specimen  known  to  contain  impurities  would  yield  nothing  of  value. 
Mitchell  County  albite  showed  signs  of  melting  after  four  hours  at 
1 100°.  Under  a  high  power  the  crystal  edges  appeared  weathered  or 
toothed — strongly  resembling  the  incipient  melting  of  the  ice  on  a 
frosted  window  pane.  These  extremely  fine  teeth  could  be  followed 


ALBITE.  51 

through  the  slide  on  exposed  edges.  At  1125°  (Plate  XIX  X  600)  a 
four  hours'  heating  gave  unmistakable  glass  in  tiny  pockets  and  lanes. 

The  above  experiments  with  the  Cloudland  albite  were  completed 
before  we  obtained  the  Amelia  County  material,  but  the  latter  proved 
to  be  so  much  nearer  to  the  type  of  pure  soda  feldspar  that  nearly  all 
the  experiments  were  repeated  with  it,  except  that  the  crystal  blocks 
were  embedded  in  powdered  crystals.  We  did  not  develop  any  new 
fact,  however ;  the  effects  noted  above  reappeared  in  the  same  order, 
except  perhaps  that  melting  went  on  a  little  faster  in  the  Amelia 
County  specimen.  As  much  melting  was  found  after  one-half  hour 
at  1200°  with  the  Amelia  County  sample  as  the  Cloudland  (Mitchell 
County)  albite  showed  in  the  same  time  at  1225°,  which  is  readily 
enough  explained  by  the  relatively  large  quantity  of  lime  (anorthite) 
in  the  latter. 

Since  both  time  and  temperature  enter  into  the  delimitation  of  the 
metastable  region,  further  trials  at  temperatures  above  1250°  did  not 
seem  likely  to  add  anything  to  the  knowledge  already  obtained.  And 
if  the  heating  were  very  rapid,  the  temperature  differences  within  the 
charge  would  be  considerable.  A  few  isolated  crystalline  fragments 
were  found  in  a  microcline  melt  which  had  been  heated  as  high  as 
1400°  for  another  purpose.  Another  which  had  reached  nearly 
1 500°  showed  no  microcline,  but  one  or  two  minute  quartz  inclusions 
still  remained  undissolved. 

We  made  a  rough  attempt  to  get  a  more  tangible  idea  of  the  viscos- 
ity of  these  feldspars  at  their  melting  temperature  in  the  following 
way:  A  long,  slender  sliver  (perhaps  30  X  2  X  i  mm.)  of  albite  and 
one  of  microcline  were  chipped  from  larger  portions,  spanned  across 
small  empty  platinum  crucibles,  and  placed  side  by  side  in  the  furnace. 
These  exposed  crystals  were  heated  to  1225°  for  three  hours.  When 
removed  they  were  completely  amorphous  (melted),  but  retained 
their  position  with  hardly  a  trace  of  sagging. 

After  this  a  number  of  similar  slivers  were  prepared,  mounted  in  the 
same  way,  and  heated  to  temperatures  of  from  1200°  to  1300°  for  a 
few  moments.  At  their  highest  temperature  a  platinum  rod  was  in- 
serted through  a  hole  in  the  top  of  the  furnace  and  allowed  to  rest  as  a 
load  upon  the  middle  of  the  crystal  bridges.  Under  this  load  the 
partially  melted  slivers  gradually  gave  way  and  were  taken  from  the 
furnace  in  the  various  forms  shown  in  the  illustrations.  Slides  cut 
from  these  showed  no  squeezing  out  of  the  melted  portion  between  the 
crystal  fragments  on  the  side  toward  the  center  of  curvature,  or  open 
cracks  on  the  outer  side  (Plates  XXIV,  XXV,  and  XXVI) .  It  will  be 
noticed  that  the  melting  began  on  the  convex  surface,  where  the 


52         ISOMORPHISM   AND  THERMAL   PROPERTIES   OF   FELDSPARS 

strain  was  greatest.  On  the  other  hand,  a  variable  extinction  angle 
in  an  unbroken  crystal  fragment  frequently  gave  unmistakable  evi- 
dence of  the  bending  of  the  crystal  as  well  as  the  vitreous  portion. 
From  these  qualitative  experiments  it  seems  possible  to  assert  with 
confidence  that  the  order  of  magnitude  of  the  viscosity  of  the  molten 
portion  (glass)  is  the  same  as  that  of  the  rigidity  of  the  crystals  at 
these  temperatures.  Plate  XXIV  shows  a  piece  of  Mitchell  County 
albite  heated  to  1200°  under  load.  The  sagging  is  indicated  by  the 
curved  cleavage  cracks.  A  sliver  of  microcline,  similarly  treated,  is 
reproduced  in  Plate  XXV.  The  displacement  is  shown  by  the  curva- 
ture of  the  crystal  edges  and  the  cleavage  cracks ;  the  black  portions 
are  glass.  It  is  interesting  to  observe  that  while  the  crystal  has 
melted  completely  across,  there  has  been  no  displacement  of  the 
cleavage  plane  (indicated  by  a  dotted  line). 

Plate  XV  is  from  a  charge  of  composition  Aba  An  i  which  had  been 
heated  to  1375°  and  completely  melted.  It  was  then  allowed  to  cool 
slowly  in  the  furnace.  On  the  following  day  it  was  reheated  to  about 
1 250°  for  most  of  the  day.  The  slide  was  made  from  this  mass.  The 
dark  portions  of  the  slide  are  glass  in  which  the  crystals  were  induced 
by  the  subsequent  reheating.  At  first  sight  it  would  seem  that  crys- 
tallization ought  to  be  complete  after  the  mass  had  been  allowed  to 
cool  in  the  furnace  and  had  been  reheated  for  six  hours  at  a  tempera- 
ture within  125  degrees  of  its  melting  point,  but  the  slide  plainly 
shows  that  equilibrium  is  reached  very  slowly  in  melts  of  this  extreme 
viscosity,  even  after  nuclei  have  formed. 

The  preceding  experiments  gave  a  clear  idea  of  the  phenomena 
attending  the  melting  of  albite  and  orthoclase,  and  convinced  us  that 
the  absorption  .of  heat  accompanying  fusion,  which  we  had  searched 
for  in  vain  upon  the  heating  curves  in  the  earlier  experiments,  had 
eluded  us  merely  because  it  was  extended  over  so  long  a  stretch  of  the 
curve  as  not  to  be  noticeable.  Some  very  exact  measurements  of  the 
temperature  change  from  minute  to  minute  were  therefore  made  in 
the  hope  that  a  more  intelligent  search  might  be  more  successful. 
Separate  charges  of  glass  and  of  crystals  of  the  same  composition  and 
of  equal  weight  were  prepared  and  successively  heated  in  the  same 
furnace  with  the  same  current.  The  specific  heat  is,  of  course,  not 
identical  in  the  two  cases,  but  the  curves  were  comparable  in  form. 
Above  1100°  we  felt  sure  that  one  of  the  curves  must  contain  an 
absorption  of  heat  which  would  be  absent  from  the  other.  Such  a 
pair  of  curves  (I),  taken  from  the  microcline  measurements,  is  repro- 
duced in  the  adjoining  figure  (fig.  12),  and  appears  to  show  such  an 
absorption  clearly,  extending  from  1135°  to  1275°.  The  dotted  line 


53 


C   '1500 


11000 

0.35') 


10000 


9500 
(1000°) 


t 


Time  -  1  division  =  10  minutes 
FIG.  12. — Curves  showing  the  absorption  of  heat  in  melting  orthoclase. 

shows  the  course  of  the  curve  without  the  absorption,  as  inferred  from 
the  glass  curve.  The  same  figure  contains  two  other  curves  (II,  III), 
similarly  obtained,  which  were  made  upon  fresh  charges  of  the  same 


54          ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

material  but  with  different  rates  of  heating.  It  will  be  noticed  that 
the  absorption  begins  to  be  noticeable  at  a  slightly  lower  temperature 
if  the  heating  is  slower. 

This  peculiar  behavior  shown  by  compounds  which  melt  to  form 
hyperviscous  liquids  seems  not  to  have  been  observed  before  and  to 
contain  features  of  more  than  ordinary  interest.  Here  are  evidently 
crystalline  substances  which  not  only  can  exist  for  considerable  peri- 
ods of  time  at  temperatures  far  above  their  melting  temperatures, 
but  which  melt  with  extreme  slowness  in  the  lower  portion  of  this 
range  of  instability.  It  would  certainly  be  no  exaggeration  to  say 
that  the  albite  with  which  we  worked  would  require  some  weeks  to 
reach  the  amorphous  state  if  maintained  at  a  constant  temperature 
of  1125°. 

An  interesting  question  arises  here  as  to  the  state  of  the  crystalline 
material  at  temperatures  above  its  melting  point.  It  is  easily  con- 
ceivable that  the  crystals  are  merely  superheated  without  loss  of  any 
of  their  properties  as  solids,  and  that  they  thus  present  an  analogy  to 
superheated  liquids.  In  the  transformation  (Umwandlung)  of  a  solid 
crystalline  substance  into  another  crystal  form  such  superheating  has 
long  been  known.  The  change  is  dependent  upon  temperature  and 
pressure  like  ordinary  fusion,  but  it  is  possible  to  pass  the  transforma- 
tion temperature  in  either  direction.  This  must  be  due  to  the  unfa- 
vorable opportunity  for  molecular  motion  which  solids  afford,  and  the 
latter  should  differ  in  no  essential  particular  from  ultraviscosity. 

On  the  other  hand,  it  does  not  seem  a  violation  of  any  known  prin- 
ciple to  conceive  cases  of  unstable  equilibrium  in  which  the  molecules 
of  a  liquid  are  oriented  as  in  a  crystal.  Maxwell's  demons  might 
arrange  them  much  like  a  school  of  fish,  and  there  is  no  apparent  reason 
why  the  fluidity  should  be  destroyed  thereby.  Were  such  an  arrange- 
ment one  of  minimum  potential,  the  mass  would  be  a  liquid  crystal. 
In  the  supposed  case  such  a  substance  would  possess  a  melting  point 
dependent  upon  the  temperature  and  pressure  above  which  Maxwell's 
definition*  of  a  true  solid — that  its  viscosity  be  infinite — would  no 
longer  obtain,  although  deorientation  might  not  become  apparent,  in 
the  face  of  extreme  viscosity,  for  a  considerable  time  afterward .  Such 
a  melting  point  would  be  determinable  only  with  the  greatest  diffi- 
culty, for  all  the  functions — mechanical,  thermal,  or  electrical — which 
usually  become  suddenly  discontinuous  at  the  melting  point  would 
be  equally  powerless  to  define  a  change  of  state  in  the  face  of  such 
extreme  molecular  inertia. 


*  Maxwell's  Scientific  Papers,  vol.  2,  p.  620 


SPECIFIC  GRAVITY.  55 

In  substances  like  these,  which  we  found  to  be  still  viscous  at  the 
temperature  of  the  electric  arc,  the  sharpness  of  a  minimum  due  to 
heat  absorption,  for  example,  is  not  dependent  upon  the  magnitude 
of  that  absorption  entirely,  but  also  upon  the  rapidity  with  which  the 
change  which  involves  it  proceeds.  In  albite  and  orthoclase  the 
velocity  of  this  change  is  very  small. 

SPECIFIC  GRAVITY. 

The  study  of  the  specific  gravities  yielded  one  interesting  result 
which  was  not  anticipated.  The  artificial  feldspars,  being  chemically 
pure  and  homogeneous,  gave  a  perfectly  definite  specific  gravity  which 
could  be  determined  with  great  accuracy  if  the  specimen  was  com- 
pletely crystallized.  If  vitreous  inclusions  were  still  present,  the 
results  were  of  course  variable  and  were  all  too  low.  It  was  antici- 
pated that  the  specific  gravity  of  pure  glasses,  even  when  transparent 
and  free  from  bubbles,  as  they  were  in  the  more  calcic  members  of  the 
series,  might  yield  values  varying  more  or  less  with  the  rate  of  cooling, 
or  after  annealing,  but  this  did  not  prove  to  be  the  case.  Our  results 
did  not  vary  more  than  two  units  in  the  third  decimal  place  in  the  same 
preparation,  even  with  the  more  calcic  feldspars,  which  required  to  be 
very  rapidly  chilled  in  order  to  cool  the  melt  without  crystallization. 

The  determination  of  specific  gravities  is  a  trite  subject,  but  we  have 
found  the  common  methods  liable  to  such  grave  errors  that  we  ven- 
ture to  give  some  useful  details.  The  error  due  to  the  evaporation  of 
water  about  the  stopper  of  the  picnometer  is  very  much  less  with 
finely  ground  stoppers  than  with  coarse  grinding,  and  if  the  stopper 
is  slightly  vaselined  just  before  the  final  weighing  the  error  from  this 
cause  will  hardly  affect  the  third  decimal  place  with  25  cc.  picnometers. 
The  simplest  form  of  flask  with  a  small  capillary  opening  in  the 
stopper  is,  in  our  judgment,  far  superior  to  one  carrying  a  ther- 
mometer. The  temperature  should  be  made  sure  by  the  use  of  the 
thermostat. 

For  removing  the  air  from  a  powdered  charge,  we  used  the  device  of 
G.  B.  Moore,*  slightly  modified,  as  indicated  in  the  accompanying 
sketch  (fig.  13).  The  bulb  A  contains  boiled  water.  When  the  appa- 
ratus is  exhausted,  the  water  is  allowed  to  flow  back  into  the  picnom- 
eter containing  the  charge,  then  by  tapping  and  warming  with  water 
at  40°  to  50°  to  produce  boiling  within,  the  air  is  effectively  removed. 
The  material  projected  from  the  flask,  if  the  boiling  is  violent,  is  then 
washed  back  from  the  tube  B  with  boiled  water,  and  any  small  particles 


*  G.  E.  Moore,  Journ.  prakt.  Chem.,  2,  319,  1870. 


56  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

remaining  are  washed  into  a  tared  dish  and  finally  weighed.  It  is 
very  important  that  not  the  smallest  grain  of  material  should  get  into 
the  ground  joint  between  the  neck  and  the  stopper  of  the  picnometer. 
To  obviate  this,  wipe  out  the  neck  with  filter  paper  before  stoppering 
and  burn  the  paper  in  the  tared  dish.  If  the  powder  is  very  fine,  it  is 
advisable  to  allow  the  filled  picnometer  to  stand  for  some  hours  in  the 
thermostat  in  order  that  suspended  material  may  settle.  With  a 
25  cc.  picnometer  and  5  to  10  grams  of  material,  this  method  usually 
yields  concordant  results  to  the  third  decimal  place,  and  the  error  from 
all  causes  should  never  be  greater  than  2  units  (±  i)  in  the  third  place. 


Aspirator 


FIG.  13. — Apparatus  for  specific  gravity  determination. 

A  determination  of  this  accuracy  is  of  course  subject  to  a  correction 
for  buoyancy,  and  all  the  numbers  which  follow  have  been  thus 
corrected. 

There  is  another  error  to  which  accurate  specific  gravity  determina- 
tions upon  powdered  minerals  will  be  subject  unless  suitable  precau- 
tion is  taken.  The  exposure  to  the  air  during  the  period  of  grinding 
the  samples  gives  opportunity  for  the  condensation  of  sufficient  atmos- 
pheric moisture  upon  the  grains  to  affect  the  weight  in  air.  The 
amount  varies  measurably  with  the  size  of  the  grains,  as  will  be  seen 
from  the  accompanying  data,  and  probably  with  the  degree  of  satura- 
tion of  the  atmosphere  and  the  time  of  exposure. 


SPECIFIC  GRAVITY. 


57 


DETERMINATION  OF  MOISTURE  IN  i  GRAM  OF  POWDERED  MINERAL 

UPON  EXPOSURE  TO  THE  AlR. 


Mineral. 

Fineness  (mesh). 

Moisture. 

Orthoclase  (natural  glass)  
Abj  Ann  (artificial  glass)  
AbjAnj  (artificial  crystal).  .  .  . 
Abj  An!  (artificial  glass)  
Abj  Ant  (artificial  crystal)  .... 
Ab  (natural  crystal)    

<i50 
Selected,  coarse 
<C  100  >  1  20 
<C  ioo^>  120 

<   100  >   120 

Coarse 

Gram. 

o  .  006  i 

.OOOO 
.OOIO 
.0007 
.0010 
OOO6 

Do  

<^  150 

.0060 

Orthoclase  (natural  crystal)  .... 
Do.       (same  sample)  
Do.       (same  sample).  ..... 

<  120  >  150 
<  150 

Still  finer. 

.0011 

.0031 
.0059 

Orthoclase  (artificial  glass)  .... 
Do.       (portion  of  same.)  .  . 

Everything  <  100 
>  150 

.0065 

.0022 

=  finer  than. 


=  coarser  than. 


In  the  last  two  groups,  note  that  the  moisture  in  graded  portions  of 
the  same  sample  varies  with  the  fineness. 

We  also  verified  the  conclusion  of  Bunsen*  that  this  adsorbed  mois- 
ture is  not  removed  at  temperatures  only  slightly  above  100°,  but 
requires  600°  to  800°  —  equivalent  to  a  low  red  heat.  Several  samples 
for  which  the  moisture  had  been  determined  were  laid  away  in  corked 
test-tubes  for  a  number  of  weeks,  after  which  redetermination  gave 
exactly  the  former  value. 

It  is  worth  noting  in  this  connection  that  these  measured  quantities 
of  adsorbed  water  are  of  the  same  order  of  magnitude  as  those  usually 
obtained  for  the  water  content  in  feldspar  analyses,  f  where  again,  of 
course,  the  finer  the  sample  is  ground  for  the  analysis  the  greater  the 
possible  error  from  this  cause.  It  may  be  that  a  part  and  occasionally 
all  of  the  moisture  usually  found  in  these  analyses  is  adsorbed  and  the 
significance  of  its  presence  there  mistaken. 

The  number  of  feldspars  of  which  specific-gravity  determinations 
could  be  made  was  limited  only  by  the  possibility  of  obtaining  com- 
plete crystallization  within  a  reasonable  time.  Thus  Ab2Ani  was 
reheated  many  times  before  a  constant  value  was  reached.  Ab3Ani 
required  1  7  days  and  Ab4Ani  was  not  completely  crystallized  in  any  of 
our  attempts.  Crystalline  albite  was  produced  under  other  condi- 
tions. 


*  Wied.  Ann.,  24,  p.  327,  1885. 

t  Dana,  System  of  Mineralogy,  6th  ed.,  pp.  314  et  seq. 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


The  specific  gravities  of  the  glasses  and  of  so  many  of  the  crystalline 
mixtures  as  we  could  obtain  are  tabulated  below: 

SPECIFIC  GRAVITIES  OF  ARTIFICIAL  CRYSTALLINE  FELDSPARS. 

[Determinations  in  duplicate  are  braced  together.] 


An.          i   AthAns. 

AbiAn2. 

AbiAnL 

Ab^Anj. 

Ab3An!. 

Ab. 

First  de- 
termina- 
tion. 

Corrected. 

(2.764 
12.765 

02.767 

J2-734 
12.734 

a)2.732 

'2.732 
02.734 

•2.710 
(2.708 

J2.7io 
u  '2.710 

(2.680 
(2.680 

b\2'6^ 
(2.677 

f  2  .  660 
\   2.660 
(  2  .  660 

M2^9 

(  2  .  660 

(2.650 

1  2  .  648 

<  2.620 
'  2.620 

dj2'6,14 
(2.6l5 

e  2  .  604 

1 
, 
2  .607 

2.601 

/2.6I2 
g  2  .  605 

Mean  2.765 

2-733 

2.710 

2.679 

2.660 

2.649 

....               2  .  605 

SPECIFIC  GRAVITIES  OF  FELDSPAR  GLASSES. 

(  2  .  700 
(  2  .  700 

(2.647 

"l  2  .  649 
(  2  .  648 

"i  2  .  647 
^2.648 

\  2  .  649 

<  2  .  647 
(2.647 

<2-593 
(  2  .  594 

^2.591 
(2.591 

^(2.590 
(2.588 

52.533 
(2-534 

(2.482 
1  2  .  482 

02.485 

(2.458 

\  2  -  459 

/2.383 

12.382 

•       

.... 

.... 

Mean  2.700 

2.648            2.591 

2-533 

2.483 

2-458 

2.382 

a  Another  preparation. 

b  Same  material  reheated  for  several  days  at  temperatures  about  150°  below  the  melting 
point. 

c  Contained  about  0.8  per  cent  of  sodium  tungstate. 

d  Purified  by  warming  with  dilute  hydrochloric  acid,  then  with  water,  and  afterwards  with 
ammonia. 

e  Purified  by  fusion  with  acid  sodium  sulphate. 

/Assuming  the  residual  tungsten  to  be  present  as  Na2\VO4. 

{/Assuming  the  residual  tungsten  to  be  present  as  WO3. 

SINTERING. 

Incidental  to  this  work  upon  the  relation  between  the  feldspars,  we 
made  a  great  many  observations  upon  the  sintering  of  powdered  min- 
erals, both  crystalline  and  vitreous,  of  natural  and  artificial  composi- 
tion. While  the  results  have  not  enabled  us  to  offer  positive  conclu- 
sions of  importance,  they  are  worth  a  note  in  passing.  Powdered 
glasses  sinter  slowly  or  rapidly  several  hundred  degrees  below  the  melt- 
ing temperature  of  crystals  of  the  same  composition.  When  the  vis- 


SINTERING.  59 

cosity  is  relatively  small  (anorthite)  crystallization  begins  at  a  low 
temperature  and  proceeds  very  rapidly,  the  sintering  probably  being 
clue  to  the  interweaving  of  the  crystal  libers  during  their  formation. 
In  viscous  glasses  (albite)  sintering  also  begins  at  very  low  tempera- 
tures— the  finer  the  powder  and  the  slower  the  heating,  the  earlier  the 
first  traces  appear.  Long-continued  heating,  even  at  comparatively 
low  temperatures,  yields  a  perfectly  continuous  cake  (except  for  the 
included  bubbles)  the  surface  area  of  which  constantly  tends  toward  a 
minimum.  There  is  no  doubt  that  the  sintering  of  powdered  glasses  is 
due  to  flow  in  the  undercooled  liquid  and  is  a  phenomenon  in  viscosity 
and  surface  tension.  All  the  feldspar  glasses  sintered  readily  between 
700°  and  900°,  depending  on  the  fineness  of  the  powder  and  the  time. 

Powdered  crystalline  feldspars  do  not  sinter  readily  below  their 
melting  temperature.  Indeed,  we  were  at  first  inclined  to  the  view 
that  when  only  pure,  dry,  stable  crystals  are  present  they  do  not 
sinter  at  all,  however  finely  they  may  be  powdered.  We  observed  the 
phenomenon  in  natural  albite  at  1000°,  but  the  crystals  were  not 
wholly  free  from  inclusions  which  may  have  caused  chemical  reactions 
resulting  in  cementation.  Crystalline  fluorite  also  sinters  300°  below  its 
melting  temperature,  but  here  we  were  able  to  establish  a  decomposi- 
tion ;  acid  fumes  were  evolved  during  the  experiment,  and  the  sintered 
product  contained  i  per  cent  of  free  lime.  Our  final  experiments  with 
long-continued  heating  for  specific-gravity  determinations,  however, 
showed  that  the  purest  feldspars  which  we  could  prepare,  even  after 
they  had  reached  their  maximum  density,  still  sinter  very  slowly.  Thus 
AbiAn5  powder,  which  was  shown  by  a  determination  of  its  specific 
gravity  to  be  holocrystalline,  formed  a  compact  chalky  mass  in  four 
hours  at  a  temperature  about  1 50°  below  its  melting  point ;  in  three 
days  the  cake  was  as  hard  as  porcelain.  Other  feldspars  showed  the 
same  behavior.  It  is  hardly  possible  that  inhomogeneities  sufficient  to 
produce  diffusion  between  portions  of  different  concentration  could 
have  existed  in  these  charges.  There  is  considerable  indication  that 
some  of  the  crystalline  nuclei  grow  at  the  expense  of  others — perhaps 
through  exceedingly  slow  sublimation — which  may  account  for  it. 

We  made  repeated  attempts  to  locate  some  fixed  sintering  point 
which  should  be  characteristic  of  a  particular  material  by  means  of 
continuous  measurements  of  the  electrical  conductivity,  but  they 
all  indicated  that  no  such  point  exists.  The  conductivity  of  a 
dry  powder  increases  enormously  after  sintering  begins  and  would, 
therefore,  seem  to  offer  a  most  sensitive  test,  but  the  phenomenon  is 
altogether  gradual,  even  with  a  crystalline  feldspar  containing  only  a 
small  percentage  of  glass.  We  purpose  to  extend  these  observation? 
to  other  substances. 


6o 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


CONCLUSIONS. 

It  now  remains  for  us  to  gather  the  results  together  and  to  draw 
such  conclusions  as  they  appear  to  justify. 

(i)  If  the  melting  points  are  now  plotted  in  a  system  of  which  they 
form  the  ordinates,  while  the  percentage  compositions  of  the  different 
feldspars  form  the  abscissas  (fig.  14),  we  discover,  within  the  limits  of 
accuracy  of  possible  measurement  at  these  temperatures,  a  nearly 
linear  relation ;  the  melting  point  varies  very  closely  with  the  compo- 
sition. We  have  no  maximum,  no  minimum,  no  branching  of  the 
curve,  but  from  each  fusion  there  separates  a  solid  phase  of  the  same 


1600 

1500 

M 
<u 

* 
•OJB  1400 
V 

•o 
c 

<u  1300 

3 
(O 

Q. 

E   1200 
« 
1- 

1100 

/ 

An  1 
Ab 

- 

^  

—  -^ 



1  . 

^-~. 

-®  —  . 

""""-- 

--«-. 

-~  

~"    •« 

^ 

a-^. 

^* 

^ 

">N. 

.... 

^•^. 

^n               Ab,An5                 Ab.An^              Ab,  An,               Ab2An,    Ab3An, 
30                    84.1                       68.0                      51.5                       34.7          £6.1                                         0    An 
0                       15-9                        3Z.O                       48.5                        65.3          73.9                                         100  Ah 

Percentage    composition 
FIG.  14.— Curve  of  melting  temperatures  of  the  soda-lime  feldspars. 

composition  as  the  vitreous  matrix.  In  Abi  An5  it  will  be  remembered 
that  this  was  proved  by  the  separation  and  analysis  of  the  two  phases ; 
in  AbiAn2  partial  crystallization  was  accomplished  in  the  first  cooling 
and  the  remainder  in  a  subsequent  reheating  and  cooling,  the  two 
groups  of  crystals  proving  optically  identical;  a  small  quantity  of 
Ab4An!,  which  admits  of  absolute  identification  optically,  was  crystal- 
lized out  of  a  melt  of  that  composition  and  readily  recognized.  More- 
over, evidence  to  show  that  the  same  phase  always  separated  was 
likewise  presented. 

Stated  in  this  way,  the  relation  appears  to  be  a  simple  additive  one 
in  which  liquid  and  solid  phases  of  like  composition  are  stable  in  all 
proportions  of  the  components  and  behave  like  a  series  of  separate 
feldspars.  But  as  soon  as  we  consider  it  with  reference  to  the  laws  of 
solution  and  the  phase  rule,  it  can  not  be  explained  in  this  simple  way. 


CONCLUSIONS.  6 1 

First  of  all,  the  phase  rule  tells  us  at  once  that  we  can  have  no  true 
compound  here  between  the  components  albite  and  anorthite,  for  such 
a  compound  would  mean  one  more  component  and  an  additional  phase 
in  every  solution  before  equilibrium  could  be  established.  Moreover, 
if  the  mixture  had  been  eutectic  in  character,  the  component  (albite  or 
anorthite)  which  happened  to  be  in  excess  would  have  crystallized  out 
in  each  case,  causing  a  continual  change  in  the  composition  of  the  re- 
maining glass  until  the  eutectic  proportion  was  reached  and  the  result- 
ing charge  would  have  contained  only  crystals  of  one  (or,  in  case  of 
hysteresis,  both)  of  the  components  and  the  eutectic.  Our  curve  is 
continuous  and  the  resulting  charges  homogeneous  for  all  proportions 
of  the  components.  Lane's  suggestion*  that  the  triclinic  feldspars 
form  a  eutectic  series  in  which  the  eutectic  proportion  is  at  or  near 
Ab2An3  is,  therefore,  not  borne  out  by  our  experiments. 

Laying  aside  the  eutectic  mixture,  and  passing  over  to  solutions  of 
components  which  are  miscible  in  many  or  all  proportions,  we  find  a 
small  number  of  examples,  chiefly  organic  compounds,  which  have 
been  studied  as  types  by  Roozeboom,  Kiister,  Bodlander,  Garelli, 
Bruni,  Van  Eyk,  and  others,  among  which  our  series  appears  to  fall. 

APPLICATION  OF  THE   LAWS  OF  SOLUTIONS. 

From  the  physico-chemical  standpoint,  the  case  we  now  have  in 
hand  closely  resembles  Kiister ' s  problem  of  1 89 1 .  f  His  measurements 
were  made  upon  mixtures  of  organic  compounds  of  low  melting  point, 
while  ours  reached  a  maximum  temperature  of  1532°,  but  we  have, 
between  albite  and  anorthite,  an  exactly  similar  series  of  solid  solutions 
the  melting  pointsj  of  which  change  in  nearly  linear  relation  to  the 
percentage  of  the  two  compounds  which  enter  into  their  composition. 

This  simple  linear  relation  was  called  by  Kiister  perfect  isomor- 
phism, and  he  formulated  the  ' '  Rule  "  which  has  since  borne  his  name, 
that  the  solidifying  point  of  an  isomorphous  mixture  lies  on  a  straight 
line  joining  the  melting  points  of  the  components  and  can  be  calcu- 
lated from  the  percentage  composition  of  the  mixture.  If  this  line 
proved  to  be  slightly  concave  or  convex,  as  it  did  in  most  cases, 
imperfect  isomorphism  was  assigned  as  the  cause.  To  this  rule  an 

*  Lane,  Journal  of  Geology,  xn,  2,  p.  83,  1904. 

t  F.  W.  Kiister,  Zeitschr.  fur  Phys.  Chem.,  8,  p.  577,  1891. 

|  Kiister  measured  solidifying  points,  but  we  have  pointed  out  above  that  such 
measurements  lead  to  no  positive  result  in  liquids  of  such  viscosity  as  the  feld- 
spars, in  which  equilibrium  is  not  established  during  solidification.  Undercooling 
rarely  appeared  at  all  in  Kiister's  cases. 


62 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


objection  was  raised  by  Garelli*  and  elaborated  by  Bodlanderf — if  the 
solid  solution  behaves  like  other  solutions,  a  small  quantity  of  com- 
ponent B  added  to  component  A  can  only  lower  the  solidifying  point 
of  A  when  the  solid  phase  is  richer  in  A  than  the  liquid  phase.  The 
reasoning  is  this  (Bodlander):  Let  x\  (fig.  15)  be  the  vapor- tension 
curve  of  component  A  in  the  liquid  state,  y\  the  solidifying  point  (ti) 
of  A,  and  z\  the  vapor-tension  curve  of  solid  A .  Now,  if  a  small  quan- 
tity of  B  is  added  and  the  solid  phase  which  crystallizes  out  contains 
the  same  proportions  of  A  and  B  as  the  liquid  mixture  in  which  it 
formed,  the  vapor  tensions  of  the  liquid  and  solid  phases  must  have 
been  lowered  equally  and  the  solidifying  point  will  fall  at  y2  with  the 
same  temperature  as  the  pure  solvent.  (Equality  of  vapor  tension 


Temp. 


FIG.  15. 


100  A  -*— Composition 
FlG.   16. 


100  B 


in  the  solid  and  liquid  phases  determines  the  temperature  of  change  of 
state.)  If  A  crystallizes  alone  from  A  -{-B,  the  vapor-tension  curve 
will  continue  on  to  22  and  the  temperature  of  solidification  fall  to  t2 ; 
while  if  the  solid  phase  contains  both  components  but  is  richer  in  A 
than  the  liquid  phase,  solidification  will  occur  at  an  intermediate  point. 

Fig.  1 6  will  serve  to  show  the  crucial  character  of  the  issue  raised. 
The  ordinates  represent  temperatures  and  the  abscissas  percentages 
of  A  and  B.  Kiister  finds  his  solid  and  liquid  phases  identical  in 
composition  within  the  limits  of  experimental  error  and  the  solidify- 
ing temperature  on  the  line  A  B  at  a  point  which  can  be  determined 
from  the  proportions  of  the  components — at  d  for  example.  But  the 
laws  of  dilute  solutions  tell  us  that  if  the  phases  are  identical  in  com- 
position the  solidifying  point  of  A  -f-  B  must  fall  at  c,  i.  e.,  must 
remain  the  same  as  for  pure  A . 

The  temperatures  at  which  Kiister 's  observations  were  made  and 
their  painstaking  character  leave  no  doubt  as  to  the  validity  of  the 


*  F.  Garelli,  La  Gazzetta  Chimica  Italiana,  xxvi,  p.  263,  1894. 
f  Bodlander,  Neues  Jahrb.  f.  Min.,  Beilage,  Bd.  xii,  p.  52,  1899. 


CONCLUSIONS.  63 

experimental  fact.  Neither  can  it  be  objected  that  Kuster's  solutions 
were  not  sufficiently  dilute  to  reveal  the  true  relation,  for  the  observa- 
tions upon  naphthaline  and  /2-naphthol  have  been  repeated  by 
Bruni*  with  very  dilute  solutions  of  one  of  the  components  in  the 
other,  and  completely  verified. 

Now,  the  laws  of  solutions  hold  for  solid  solutions  even  for  moder- 
ately high  concentrations  (Bodlander)  when  the  components  are  not 
isomorphous,  and  on  the  other  hand,  even  liquid  crystals,  when  iso- 
morphous,  follow  Kuster's  rule  more  nearly  than  the  law  of  solutions. 

An  extended  discussion  of  existing  data  from  this  standpoint  would 
involve  us  in  unnecessary  detail ;  but  there  can  be  no  question  that 
Kuster's  rule  represents  the  data  which  have  been  gathered  upon 
isomorphous  mixtures — at  least  approximately — while  the  laws  of 
dilute  solutions  appear  to  fail  of  application  there.  On  the  other 
side,  the  rule  admits  of  no  independent  theoretical  derivation.  Van't 
Hofff  suggests  that  judgment  be  suspended  pending  the  accumula- 
tion of  further  data  and  intimates  that  the  close  similarity  of  chemi- 
cal composition  and  molecular  structure  in  compounds  which  form 
isomorphous  mixtures  gives  them  an  unusually  close  inter-relation, 
and  their  influence  one  upon  the  other  may  render  a  simple  theo- 
retical treatment  very  difficult. 

Our  case  is  especially  interesting  when  considered  from  this  stand- 
point, but  it  distinctly  emphasizes  the  difficulty  rather  than  helps 
toward  its  solution:  (i)  Although  the  chemical  reactions  of  albite 
and  anorthite  are  not  of  such  a  character  as  to  prove  or  disprove  a 
close  analogy  between  them,  a  comparison  of  their  formulae  certainly 
does  not  suggest  an  isomorphous  relation.  If  their  formula  weights 
represent  true  molecules,  they  possess  the  same  number  of  atoms  to 
the  molecule  (NaAl  SisOs,  CaAl2  Si2Os)  and  the  group  Si2O8  in  com- 
mon, but  the  remaining  atoms  taken  separately  are  not  mutually  re- 
placeable. (2)  The  melting  points  of  the  components  in  the  feldspar 
series  are  very  far  apart — more  than  300° — while  Kuster's  organic 
mixtures  were  all  included  within  a  narrow  temperature  interval 
(2°  to  56°).  For  reasons  which  will  appear  presently,  both  Garellij: 
and  Roozeboom  have  pointed  out  that  the  farther  apart  the  melting 
points  of  the  components  the  less  probable  is  the  linear  relation. 
(3)  The  homogeneity  of  the  solid  phase  is  established  within  i  per 
cent  by  the  optical  examination  of  the  slides.  Moreover,  separate 
chemical  analyses  of  the  solid  and  liquid  phases  of  the  mixture  Abi  An5 

*G.  Bruni,  Atti  della  reale  Accademia  dei  Lincei,  5,  vn,  p.  138,  1898. 
f  Van't  Hoff,  Vorlesungen  iib.  Theoret.  u.  Phys.  Chem.  (Braunschweig,  1901). 
Part  II,  p.  64. 

$  F.  Garelli,  loc.  cit. 


64  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

in  an  exceptionally  favorable  case  showed  still  closer  identity  of 
composition. 

It  appears  altogether  improbable  that  the  laws  of  solutions  can 
apply  in  the  face  of  so  extreme  a  controverting  case. 

If  it  has  proved  difficult  to  bring  the  isomorphous  mixture  within 
the  general  laws  of  solutions,  a  most  satisfactory  theoretical  deriva- 
tion of  the  conditions  of  equilibrium  in  such  mixtures  has  been 
developed  by  Roozeboom.  No  other  principle  is  required  than  the 
second  law  of  thermodynamics  as  applied  to  solutions  by  Gibbs: 
A  system  of  substances  will  be  in  equilibrium  for  a  particular  pres- 
sure when  the  thermodynamic  potential  (C-function)  of  the  system 
is  a  minimum.  The  scheme  of  representation  is  the  graphical  one 
proposed  by  Van  Ryn  Van  Alkemade,*  and  is  itself  a  powerful 
instrument  of  analysis  in  this  field. 

*Zeitschr.  f.  Phys.  Chem.,  u,  p.  289,  1893. 

Except  for  the  suggestions  of  Vogt  to  which  reference  has  been  made,  this 
method  seems  not  to  have  been  utilized  for  the  study  of  mineral  solutions  before. 
A  brief  outline  of  it  will,  therefore,  be  given  here. 

In  a  system  of  rectilinear  coordinates  (fig.  1 7)  the  ordinates  may  represent  the 
potential  of  a  particular  system — (Gibbs'  ^-function,  not  directly  measurable)  and 

the  abscissas  the  number  of  gram-mole- 
P.  T  constant  ,  ,        ,  ,  . 

cules  of  solvent  (water   for  example) 

supposed  to  contain  i  gr.  mol.  of  solute. 
In  other  words,  every  point  of  the  curve 
represents  a  solution  of  which  the  x 
coordinate  is  concentration  and  the  y 

o          ^^r     v  coordinate  the  potential.     The  condi- 

tions of  pressure  and  temperature  are 
assumed  constant  for  a  particular  dia- 


Concentration     — — »•  gram. 

J?JQ    j  j  Every  such  curve  for  substances  solu- 

ble in  all  proportions  will   be  convex 

downward,  otherwise  there  would  be  some  particular  point  on  the  curve  which 
would  not  represent  a  minimum  potential  for  a  particular  composition  and  the 
solution  would  tend  to  separate  into  two,  the  mean  potential  of  which  would  be 
lower. 

The  condition  for  equilibrium  between  such  a  solution  and  its  solid  phase  (pure 
salt)  may  now  be  readily  found.  Lay  off  on  the  £-axis  a  distance  equal  to  the 
potential  of  the  solid  salt  and  from  the  point  so  obtained  draw  a  tangent  to  the 
curve.  This  tangent  is  the  locus  of  minimum  potential  (stable  systems)  for  any 
composition.  At  the  point  a,  for  example,  we  have  a  saturated  solution  contain- 
ing the  number  of  gr.  mol.  of  solvent  indicated  by  the  corresponding  abscissa  and 

the  proportion  — ^  of  salt,  the  balance  of  the  salt  remaining  in  solid  phase.  At  b 
we  have  the  saturated  solution  with  all  the  salt  included ;  to  the  left  of  b  upon  the 
curve,  supersaturated  solution;  and  to  the  right  unsaturated  solution.  With 
increase  of  temperature  the  form  of  the  curve  changes  and  c  approaches  d,  the 
melting  point  of  the  salt. 


CONCLUSIONS.  65 

Roozeboom  distinguishes  three  general  classes  of  isomorphous 
mixtures : 

(1)  The  components  are  miscible  in  all  proportions  from  o  to  100 
per  cent  in  both  solid  and  liquid  phases. 

(2)  Miscibility  is  limited  to  certain  concentrations. 

(3)  More  than  one  type  of  crystal  occurs. 

In  the  feldspars  we  are  concerned  with  the  first  class  only,  but  here 
also  Roozeboom  distinguishes  three  possible  types: 

Type  I. — Melting  (or  solidifying)  points  of  the  mixtures  lie  on  a  con- 
tinuous curve  joining  the  melting  points  of  the  components  and  con- 
taining neither  maximum  nor  minimum. 

Type  II. — The  curve  contains  a  maximum. 

Type  III. — The  curve  contains  a  minimum. 

These  types  are  for  the  moment  purely  hypothetical  and  are  a  prod- 
uct of  the  method  of  analysis,  though  they  are  being  rapidly  identi- 
fied for  various  isomorphous  pairs  by  pupils  of  Roozeboom  and  by 
others. 

The  method  of  reasoning  which  yields  these  three  possible  types 
will  be  briefly  described  with  the  help  of  the  Van  Alkemade  graphical 
analysis : 

If  we  indicate  the  potential  (£)  of  a  particular  mixture  by  the  length 
of  the  ordinate  (fig.  18),  and  the  number  of  molecules  of  A  and  B  by 
subdividing  the  horizontal  axis  (A  -f-  B  =  100)  in  the  proper  propor- 
tion, assuming  atmospheric  pressure  and  constant  temperature  for 
each  diagram,  then  every  point  within  the  coordinates  represents  a 
particular  phase  of  known  composition  and  potential.  Suppose,  now 
(Roozeboom),  a  temperature  is  assumed  above  the  melting  point  of 
the  higher-melting  component;  clearly,  whatever  the  composition, 
only  the  liquid  phase  can  have  a  stable  existence.  If  potential  differ- 
ence represents  the  measure  of  the  tendency  to  change  and  the 
tendency  of  all  change  is  toward  the  minimum  potential,  for  this  tem- 
perature all  change  will  be  toward  the  liquid ;  and  the  potential  of  a 
solid,  if  one  existed  there,  would  be  greater  than  that  of  the  liquid  for 
all  compositions — hence  the  curve  5  (solid)  above  the  curve  L  (liquid) 
throughout. 

Suppose  the  potential  to  be  lowered  to  a  point  where  crystallization 
can  begin.  The  tendency  to  melt  no  longer  obtains  for  all  composi- 
tions ;  the  two  curves  will  be  displaced  relatively  and,  being  of  different 
form,  will  intersect.  Draw  a  common  tangent  to  the  curves  and  apply 
Van  Alkemade's  reasoning  above  noted.  The  trend  of  the  potential 
of  both  phases  between  the  points  of  tangency,  i.  e.,  of  all  mixtures 
between  these  limits  of  composition,  is  toward  the  minimum  repre- 


66 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


sented  by  this  tangent.  Crystallization  will  then  begin  at  a  (fig.  18, 
II),  with  the  mixture  richest  in  the  higher  melting  component,  crys- 
tals of  composition  a  will  be  in  equilibrium  with  the  liquid  phase  b  in 
all  proportions,  and  solidification  (or  melting)  will  not  take  place  at  a 
single  temperature,  but  through  a  range  of  temperature.  If  we  now 


100   A 


100  B 


100  A 


100  B 


FIG.  19. 


100  A 


100  B 


100  A 


a      b  100  B 


plot  the  length  of  the  abscissa  cor- 
responding to  ab  in  a  separate  dia- 
gram with  the  observed  tempera- 
ture range  of  solidification,  adding 
all  the  other  possible  cases  which 
will  arise  from  the  continued  dis- 
placement of  the  --curves,  we  ar- 
rive at  the  accompanying  diagram 
(fig.  19)  of  Roozeboom's  Type  I. 
Types  II  and  III  appear  in  the  same 
way  when  the  form  of  the  C-curves 
changes  as  indicated  in  figs.  20 
and  21. 

The  physical  side  of  the  system  of 
reasoning  is  readily  inferred  from 
the  figures.  If  we  start  with  a  mix- 
ture of  the  composition  indicated 
by  m  (fig.  22)  and  temperature 
above  the  melting  point,  crystalliza- 
tion will  begin  at  a,  the  separating 
crystals  will  have  the  composition 
6,  while  that  of  the  remaining  melt  approaches  d.  Upon  cooling  to  e, 
solidification  ends  with  crystals  of  this  composition.  Melting  is  ex- 
actly the  reverse  operation.  Whether  these  first  crystals  of  compo- 
sition b  remain  stable  as  such  or  undergo  solid  transformation  or  wholly 
or  partly  redissolve  appears  to  remain  undetermined  in  any  general 
way  by  Roozeboom's  theory,  and  may  be  radically  influenced  by 


100    A 


100  B 


FIG.  1 8. 


CONCLUSIONS. 


100  A 


100  A     a        b 


100  A   b 


100  A 


100  B 


a    b  100  E 


100  B 


100  B 


IQO  A  at 

FIG.  20. 


IOOB 


lOOAa       b 


100  A 


IOOB 


100  B 


100  A  a  b          100  B 

FIG.  21. 


accompanying  phenomena  like  viscosity  and  undercooling.  If  a  liquid 
mixture  of  composition  a  undercools  to  e  before  crystallization  begins, 
crystals  of  composition  e  will  appear  and  no  others  (provided  the  re- 


68 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


100  A 


Composition 
FIG.  22. 


100  B 


lease  of  latent  heat  does  not  raise  the  temperature  above  e  again). 
Such  a  situation  is  certainly  unavoidable  in  viscous  mixtures  like  the 
feldspars  and  accounts  very  well  for  the  homogeneous  solidification  ob- 
served by  us.  This  would  classify  the  feldspars  with  Type  I  of  Rooze- 

boom's  series.  A  comparison  of  our 
melting-point  curve  with  figs.  19, 
20,  and  2 1  shows  this  to  be  the  only 
type  under  which  it  could  possibly 
fall.  There  is  no  trace  of  a  maxi- 
mum or  minimum  in  the  feldspar 
curve.  Vogt's  expectation  that  they 
would  fall  under  Type  III,  therefore, 
fails  of  fulfilment  from  our  experi- 
ments. 
— ""^i  P 

That  our  curve  so  closely  resem- 
bles one  branch  of  Roozeboom's 
typical  curve  is  remarkable.  The 
difficulties  of  observation  in  those 
portions  of  the  curve  where  the 
viscosity  becomes  so  disturbing  are 
too  great  to  enable  stress  to  be  laid 
upon  the  form  which  our  curve  happens  to  take  there,  but  near  the 
anorthite  end  of  the  series  its  slight  convexity  is  unquestionably  real. 
It  should  be  added  that  Professor  Iddings  has  found  slight  traces  of 
inhomogeneity  (less  than  i  per  cent)  in  the  slides  of  several  of  our 
intermediate  feldspars.  Crystals  have  been  found  which  were  evi- 
dently of  the  earliest  formation,  and  with  one  exception  were  more 
calcic  than  the  body  of  the  charge,  as  Roozeboom's  theory  would  lead 
us  to  expect.  The  exception  was  an  occurrence  of  tiny  plates  of 
Ab4Ani  discovered  in  a  charge  of  Abi  Ans.  The  extremely  small  quan- 
tity of  the  optically  different  feldspar,  the  fact  that  it  could  not  be 
found  in  all  the  slides  of  this  composition,  and  that  in  one  case  a  less 
calcic  feldspar  appeared,  suggest  that  the  inhomogeneity  may  have 
been  of  other  origin — perhaps  merely  a  consequence  of  the  tremendous 
difficulty  in  mixing  a  homogeneous  charge  where  ultraviscosity  pre- 
cludes stirring,  for  example.  The  chemical  analysis  of  the  solid  and 
liquid  phases,  it  will  be  remembered,  showed  identical  composition 
within  the  limits  of  experimental  error. 

It  is  clear  that  if  Roozeboom's  theory  is  valid,  the  line  of  the  melt- 
ing points  can  not  become  perfectly  straight  unless  the  ^-curves  for 
the  solid  and  the  liquid  phases  can  be  superposed  point  for  point 
throughout,  i.  e.,  are  identical.  This  would  mean  that  the  energy 


LITHOLOGICAI,  APPLICATIONS.  69 

content  per  gr.  mol.  of  solid  and  liquid  phase  was  the  same  for  all  com- 
positions, i.  e.,  that  all  mixtures  and  the  components  separately  should 
have  the  same  melting  point — a  case  which  is  known  (Roozeboom, 
d-  and  /-  camphor  oxime),  but  is  certainly  confined  to  optical  antipodes. 
Another  reason  for  supposing  the  case  to  be  much  less  simple  than 
a  mere  linear  relation  with  equilibrium  between  solid  and  liquid  phases 
of  identical  composition  appears  at  once  from  a  direct  application  of 
the  phase  rule.  A  necessary  condition  for  equilibrium  in  any  mixture 
is  that  the  number  of  phases  exceed  the  number  of  components  by 
two.  If  the  solid  and  liquid  phases  are  homogeneous,  the  number  of 
phases  (counting  vapor)  is  only  three,  and  equilibrium  can  not  obtain 
there. 

UTHOLOGICAL   APPLICATIONS. 

Supposing  the  case  for  the  feldspars  to  be  established,  by  this  line 
of  reasoning,  as  falling  under  Type  I  of  Roozeboom's  classification, 
important  light  is  thrown  on  the  significance  of  zonal  structure  in 
feldspars  and  also  on  the  meaning  of  its  absence.  A  very  considera- 
ble proportion  of  the  feldspars  found  in  thin  sections  of  rocks  show 
zonal  structure,  though  it  is  more  frequent  in  effusive  lavas  than  in 
the  granular  massive  rocks. 

Furthermore,  with  rare  exceptions,  the  outer  zones  are  more  sodic 
than  those  which  they  inclose.  The  width  and  definition  of  the  zones 
vary  greatly;  they  are  sometimes  sharply  separated;  not  infre- 
quently they  show  transitions  at  the  edges  of  the  zones,  and  occasion- 
ally the  gradation  is  a  continuous  one,  so  that  the  extinction  during 
a  rotation  of  the  slide  resembles  a  shadow  moving  at  a  uniform  rate. 
This  last  case  is  immediately  explicable  by  Roozeboom's  theory.  If 
a  feldspar  magma  of  any  particular  composition  were  to  solidify  with- 
out undercooling,  the  composition  would  change  continuously  during 
solidification  in  a  perfectly  definite  manner,  within  limited  ranges  of 
temperature  and  composition,  as  has  been  indicated  in  the  discussion 
of  the  theory  above,  the  center  being  always  more  calcic  than  the 
periphery. 

Homogeneous  crystals  are  also  readily  explained.  If  undercooling 
occurs  the  magma  does  not  begin  to  crystallize  until  it  has  passed 
below  the  range  of  temperature  at  which  the  change  in  concentra- 
tion can  take  place. 

Sharply  emphasized  zones,  or  zones  showing  transitions  only  at 
their  edges,  point  to  changes  in  physical  conditions  during  crystalliza- 
tion. Now  abrupt  changes  of  pressure  are  not  likely  to  be  frequent 
excepting  during  the  act  of  intrusion  or  extrusion,  but  in  complex 


70  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

magmas  there  inevitably  must  be  local  variations  in  temperature  in 
consequence  of  the  liberation  of  energy  during  the  crystallization 
of  the  feldspars  and  of  the  accompanying  mineral  constituents, 
especially  the  ferromagnesian  silicates.  When  the  "crystallization 
goes  on  slowly  and  smoothly,  the  magma  may  be  expected  to  cool 
gradually  over  the  range  between  the  two  curves  (fig.  22),  and  a 
uniform  zonal  structure  result,  the  extreme  viscosity  of  the  liquid 
operating  to  prevent  any  considerable  diffusion,  resorption,  or  other 
modifying  phenomena.  Any  sudden  or  irregular  release  of  heat 
which  tends  to  prevent  uniform  cooling,  if  it  occurs  within  the  range 
of  possible  zonal  formation,  may  be  expected  to  result  in  some  varia- 
tion in  the  bands;  constant  temperature  for  a  considerable  interval 
will  tend  to  produce  broad  bands  of  uniform  composition  through  the 
resorption  of  more  calcic  crystals  already  formed,  and  sharp  demarka- 
ations.  In  fact,  any  considerable  disturbance,  either  mechanical  or 
thermal,  would  probably  result  in  sharp  demarkations  between  bands. 
Again,  if  the  temperature  change  should  carry  the  crystals  below  this 
critical  region,  only  homogeneous  crystals  would  form  unless  a  near-by 
release  of  heat  could  raise  it  again.  It  is  easily  conceivable  that  this 
latter  case  might  produce  a  partial  reversal  of  the  order  of  the  bands. 

In  a  word,  if  feldspar  crystals  begin  to  form  within  the  range  where 
a  change  in  concentration  can  occur,  zonal  structure  will  probably 
result,  and  every  change  in  the  temperature  will  have  its  effect  upon 
the  arrangement  of  the  zones.  Long-continued  freedom  from  ther- 
mal disturbance  will  produce  broad  zones,  and  rapid  variation,  either 
continuous  or  irregular,  will  produce  narrow  or  sharply  bounded  ones. 

Inversely,  it  would  appear  that  whenever  a  set  of  thin  sections 
shows  traces  of  zonal  structure,  and  there  are  few  hand  specimens  in 
which  this  structure  can  not  be  detected,  solidification  of  the  feldspars 
has  taken  place  within  well-defined  limits  of  temperature.  When,  as 
in  the  granites,  water  vapor  or  its  components  have  entered  into  the 
composition  of  the  magma,  it  is  probable  that  this  range  of  tempera- 
ture is  a  different  one,  a  point  to  be  determined  by  further  researches, 
but  it  is  evidently  practicable  to  determine  for  granites  as  well  as  for 
the  nearly  anhydrous  lavas  at  what  temperature  the  feldspars  have 
solidified,  wherever  zonal  structure  can  be  found. 


SUMMARY   OF   CONCLUSIONS. 


SUMMARY    OF    CONCLUSIONS. 

Reviewing  this  discussion  briefly :  (i)  The  triclinic  feldspars  are  solid 
solutions  and  form  together  an  isomorphous  series.  It  is  a  sufficient 
condition  for  the  latter  that  the  curve  of  melting  points  is  continuous 
(Bruni,  loc.  cit.).  Like  Kiister's  curves  for  organic  compounds,  the 
curve  of  melting  points  does  not  follow  Van't  Kofi's  law  of  dilute  solid 
solutions  and  does  approximate  closely  to  a  straight  line  joining  the 
melting  points  of  the  components.  The  case  appears  to  fall  under 
Type  I  of  Roozeboom's  theoretical  classification  of  isomorphous  mix- 


2.900 


2.800 


2.700 


Glass 


•J3   2.500 

D. 

in 

2.400 

2.300 


2.200 

An 

An  100 
Ab    0 


Ab,An5 
84.1 
15.9 


Ab,An2  Ab.An,  Ab2An,  Ab3An,  Ab 

68.0  51.5  34.7  26.1  0 

32.0  48.5  65.3  73.9  100 

Percentage    composition 

FiG.  23. — Curves  of  specific  gravity  of  the  feldspars  and  feldspar  glasses. 

tures,  in  which  case  the  line  can  not  become  exactly  straight  unless 
the  melting  points  of  the  components  are  nearly  or  quite  identical,  nor 
the  solidification  absolutely  homogeneous  without  reducing  the  num- 
ber of  phases  to  three  and  destroying  the  equilibrium.  The  theory 
also  accounts  for  an  absence  of  sharpness  in  the  intermediate  melting 
points  of  the  feldspars,  but  the  fact  that  this  lack  of  sharpness  culmi- 
nated in  albite  instead  of  terminating  there  shows  that  the  viscosity 
was  the  chief  factor  in  our  difficulties  from  this  cause.  Albite  was 
clearly  shown  to  melt  through  a  variable  range  of  1 50°  or  more,  while 
the  intermediate  feldspar  bytownite  (AbiAn5)  melted  almost  as 
sharply^as  anorthite,  as  one  would  expect  it  to  do  in  view  of  the  flat- 


72 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


ness  of  the  melting-point  curve  (p.  60).  The  fact  that  practically  no 
differences  of  composition  could  be  detected  in  our  melts  we  attribute 
to  the  effect  of  viscosity  and  consequent  undercooling,  which  resulted 
in  crystallization  invariably  resulting  at  much  too  low  a  temperature 
for  equilibrium  to  become  established  between  the  solid  and  liquid 
phases  at  any  stage  of  the  crystallization  process. 

(2)  When  the  specific  gravities  are  plotted,  like  the  melting  points, 
as  a  function  of  the  composition  (fig.  23),  the  isomorphism  of  the  feld- 
spars is  strongly  confirmed. 


^x 

^ 

^x- 

^ 

xX 

^ 

gX 

P 

Gla 

ss 

^ 

? 

^> 

^ 

<£. 

^ 

^ 

^ 

^—  -— 

—  —  • 

...  —  ' 

^ 

~? 

Cry; 

tals 

^--- 

riT^ 

-—  «• 

•  —  "* 

^ 

^ 

^-  —  • 

—  — 

^ 



#~~ 

^-  — 

• 

f~  —  • 

_*-——" 

\n                Ab,An5             Ab,An2            Ab.An,           Ab2An,       Ab3An,                            Ab 
30                     84.1                     68.0                   51.5                   34.7               £6.1                                     0     e 
0                        15.9                       32.0                   48.5                    65.3               73.9                                    100 
Percentage    composition 

.4200 


.4100 


.4000 


.3700 


.3600 


.3500 


AC 


FIG.  24. — Curves  of  specific  volume  of  the  feldspars  and  feldspar  glasses. 

The  curve  indicates  a  perfectly  continuous  relation  which  the  suc- 
cessful preparation  of  chemically  pure  albite  enabled  us  to  follow 
through  to  the  end.  The  order  of  accuracy  is  also  extraordinarily 
high  throughout  by  reason  of  the  chemical  purity  of  all  the  prepara- 
tions and  the  consistent  effort  made  to  obtain  complete  crystalliza- 
tion, even  with  the  more  viscous  feldspars.  Several  of  the  charges 
were  heated  for  two  weeks  or  more  consecutively,  then  removed  for  a 
determination,  then  replaced  in  the  furnace  for  another  week  in  order 
that  we  might  assure  ourselves,  from  the  consistent  reappearance  of 
the  same  value,  that  a  maximum,  and,  therefore,  holocrystallization. 
had  been  reached.  It  is  of  some  practical  importance  to  note  in  pass- 


SUMMARY   OF  CONCLUSIONS.  73 

ing  that  preparations  which  appeared  completely  crystalline  in  the 
slides  frequently  proved  not  to  have  reached  their  maximum  specific 
gravity.  It  is  very  difficult  to  detect  the  last  traces  of  glass  with  the 
microscope. 

If  our  confidence  in  these  determinations  is  justified,  the  form  of 
the  specific-gravity  curve  is  very  significant.  It  was  pointed  out  by 
Retgers*  that  if  the  isomorphous  mixture  is  merely  a  "mechanical 
aggregate"  the  volume  of  which  remains  exactly  equal  to  the  sum  of 
the  volumes  of  the  components,  then  the  specific-volume  curve  of  the 
mixtures  for  percentages  by  weight  of  the  two  components  must  be  a 
straight  line.  He  also  offers  a  number  of  isomorphous  pairs  for  which 
he  finds  the  specific-volume  curves  to  be  straight  lines,  in  support  of 
his  hypothesis  that  this  relation  is  general.  Our  values  when  plotted 
in  this  way  (fig.  24)  also  give  a  straight  line  with  maximum  varia- 
tions amounting  to  0.005,  which  is  probably  not  greater  than  the 
aggregate  error  in  the  syntheses  and  in  the  determinations  of  the 
specific  gravity. 

In  spite  of  this  apparent  corroboration,  it  does  not  seem  to  us  that 
Retgers  was  quite  justified  in  assuming  that  this  relation  is  entirely 
without  limitation.  The  temperature  at  which  the  specific  gravity 
is  determined  is  so  far  below  the  temperature  of  solidification  (in  our 
case  more  than  1000°)  that  the  density  at  25°  will  depend,  to  a  con- 
siderable degree,  upon  the  coefficient  of  expansion  of  the  material  as 
well  as  upon  composition  and  molecular  structure.  The  coefficient 
of  expansion  will,  in  general,  differ  for  different  compositions,  and  is  not, 
in  general,  a  linear  function  of  the  temperature.  Considering  Retgers's 
generalization  in  the  light  of  these  facts,  the  relation  of  the  specific 
gravities  at  25°  would  be  necessarily  continuous,  but  not  necessarily 
linear. 

The  specific  gravities  of  the  glasses  are  also  plotted  (fig.  23)  to  show 
the  divergence  from  the  line  of  the  crystals  toward  the  albite  end  of 
the  series,  i.  e.,  as  the  percentage  of  albite  increases  the  density  of 
the  glass  is  diminished  more  than  that  of  the  crystals. 

There  is  nothing  new  in  the  conception  of  isomorphism  in  the  feld- 
spars, but  the  positive  character  of  our  experimental  results  makes 
them  of  more  than  ordinary  interest  by  reason  of  the  fact  that  so 
good  authority  on  the  subject  as  Fouque  and  LeVy  has  passed  upon 
it  adversely  on  the  basis  of  optical  evidence  derived  from  artificial 
preparations.  More  recently  Violaf  has  declared  that  the  optical 
evidence  is  insufficient  to  prove  isomorphism  in  the  natural  feldspars 


*  J.  W.  Retgers,  Zeitschr.  fur.  Phys.  Chem.,  3,  p.  507,  1889. 
t  Loc.  cit. 


74 


ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 


The  melting  points  and  specific  gravities  plotted  above  are  brought 
together  in  a  convenient  table  here. 


Feldspar. 

Melting 
temperature 
(degrees). 

Specific  gravity. 

Crystals. 

Glass. 

An       

1532° 
1500 

1463 
1419 

1367 
1340 

2.765 
2-733 

2  .  7IO 
2.679 
2.660 
2.649 
2.605 

.700 

.648 
•591 
•533 
2.483 
2.458 
2.382 

AbjAn-  
At^Aiij  
AbjAtii^  

AbaAnx  

Abo  An  i 

Ab 

(3)  In  the  melting  of  albite  and  microcline  we  appear  to  have  sub- 
stantial evidence  of  a  phenomenon  which  is  unfamiliar  both  to  physics 
and  to  mineralogy.     Microscopic  crystals  of  a  homogeneous  com- 
pound, when  slowly  heated,  were  shown  to  persist  for  150°  or  more 
above  where  melting  began,  the  amorphous  melt  remaining  of  the 
same  order  of  viscosity  as  the  rigidity  of  the  crystals.     By  careful 
observation,  curves  were  also  obtained  showing  that  the  absorbed 
heat  of  fusion  was  distributed  over  this  interval. 

From  the  experimental  standpoint  a  substance  of  this  kind  can 
hardly  be  said  to  have  a  melting  point,  but  passes  gradually  from 
crystalline  to  amorphous  at  temperatures  which  can  be  considerably 
varied  by  merely  changing  the  rate  of  heating.  In  moderate  charges 
of  albite  or  orthoclase  at  atmospheric  pressure  this  melting  began  so 
slowly  that  it  was  not  possible  to  locate  even  approximately  a  lowest 
temperature  for  the  beginning  of  the  change  of  state.  As  a  matter  of 
definition,  this  minimum  temperature  above  which  melting  will  con- 
tinue (for  a  given  pressure)  more  or  less  rapidly,  according  to  the 
conditions,  is  the  "melting  point,"  whether  it  can  be  located  or  not, 
so  far  as  the  equilibrium  of  the  system  is  concerned;  and  crystals 
which  continue  to  exist  unmelted  at  higher  temperatures  appear 
to  form  a  metastable  phase,  perhaps  comparable  to  that  of  a  crys- 
talline solid  when  heated  above  the  "  Umwandlungstemperatur " 
without  immediate  change  of  crystal  form.  It  is  also  possible  that 
the  mass  is  fluid  when  heated  above  the  melting  point,  but  that 
deorientation  of  the  molecules  is  delayed  by  viscosity.  This  meta- 
stable stage  can  easily  extend  over  150°  in  albite  and  orthoclase  and 
would  persist  for  days  in  the  lower  portion  of  this  range. 

(4)  We  also  found  that  viscous  and  poorly-conducting  melts  which 
solidify  only  after  considerable  undercooling  do   not  give  constant 
solidifying  points.     The  solidifying  point  must  not  be  used,  therefore, 
without  great  caution  as   a  physical  constant;   it  bears  no  relation 


SUMMARY   OF  CONCLUSIONS.  75 

whatever  to  the  melting  point  unless  equilibrium  is  reestablished 
before  solidification  is  complete — a  condition  which  rarely  obtains  and 
often  can  not  be  produced  in  viscous  mineral  melts.  Especial  attention 
is  directed  to  this  because  of  the  importance  of  the  lowering  of  the 
solidifying  point  in  the  study  of  solutions,  and  the  possibility  of  its 
application  to  mineral  solutions  recently  suggested  by  Vogt.* 

(5)  Incidental  to  the  experimental  work  upon  the  feldspars  we  were 
able  to  establish  the  fact  that  there  are  no  differences  of  density  in 
the  feldspar  glasses  due  to  the  rate  of  cooling  which  are  greater  than 
our  errors  of  observation  (±  o.ooi).  Also  that  powdered  crystalline 
feldspars  which  are  free  from  inclusions  and  from  glass,  even  when 
very  fine,  do  not  sinter  until  melting  begins ;  powdered  glasses  of  like 
composition  sinter  readily  at  relatively  low  temperatures  (700°  to 
900°),  depending  primarily  upon  the  degree  of  pulverization.  Again, 
that  powdered  feldspars  when  exposed  to  the  atmosphere  adsorb 
moisture  in  quantities  of  an  order  of  magnitude  equal  to  those  usually 
quoted  in  analyses.  (Dana's  System  of  Mineralogy,  /.  c.).  It  is, 
therefore,  altogether  possible  that  the  significance  of  this  moisture  has 
sometimes  been  mistaken. 


PART  II. 


The  Isomorphism  and  Thermal  Properties 
of  the  Feldspars. 


OPTICAL    STUDY. 

BY 

].  P.  IDDINGS. 


LIME-SODA   FELDSPARS  CRYSTALLIZED  IN  OPEN 
CRUCIBLES  FROM  FUSED  CONSTITUENTS. 


INTRODUCTION. 

The  results  of  these  synthetical  experiments  agree  closely  in  some 
respects  while  differing  in  others.  They  agree  in  general  in  the  habit 
and  arrangement  of  the  crystals  of  the  different  feldspars  produced, 
while  differing  in  the  size  of  the  crystals  of  the  various  feldspars 
according  to  their  composition.  These  results  have  an  important 
bearing  on  the  problem  of  texture  and  granularity  in  igneous  rocks. 

First,  as  to  the  habit  of  the  feldspar  crystals  produced  from  solu- 
tion of  the  feldspar  constituents  without  admixture  of  other  material. 
So  far  as  can  be  determined  by  microscopical  study  of  the  sections,  the 
crystals  are  in  most  cases  blade-like  in  form ;  that  is,  they  are  elongated 
plates.  They  vary,  however,  from  one  extreme  to  another,  being  in 
some  cases  equidimensional  plates  of  extreme  thinness,  in  other  cases 
prisms,  elongated  in  one  direction  with  the  other  two  dimensions 
equal.  The  development  of  these  forms  takes  place  in  feldspars  of 
various  compositions,  and  appears  to  be  chiefly  a  function  of  the  rate 
of  crystallization  and  not  of  the  chemical  composition  of  the  feld- 
spar, except  as  this  modifies  the  viscosity  of  the  solution.  It  is  not 
possible  to  recognize  any  fixed  relation  between  the  habit  of  the 
crystals  and  the  composition  of  the  feldspar.  This  is,  of  course,  in 
accord  with  the  well-known  isomorphism  of  the  feldspar  group. 

The  common  mode  of  crystallization  in  these  preparations  is  that 
of  spherulitic  aggregations,  more  or  less  completely  developed  in 
spherical  forms. 

The  elements  of  the  spherulites  are  bundle-  or  sheaf -like  aggrega- 
tions of  long,  thin  blades,  which  blades  lie  nearly  parallel  to  one 
another  in  the  middle  or  narrower  part  of  the  bundle,  and  diverge  at 
the  ends  into  fan-like  or  plumose  forms.  Several  of  these  bundles  or 
blades  cross  one  another  at  the  middle,  and  when  there  are  a  sufficient 
number  of  bundles,  or  when  they  diverge  sufficiently,  a  completely 
spherulitic  aggregation  results. 

In  some  cases  a  spherulite  consists  of  bundles  or  prisms  that  extend 
uninterruptedly  from  the  center  to  the  outer  margin,  the  rays  of  the 
spherulite  being  nearly  straight.  In  other  cases  the  spherulite  is  a  com- 

79 


80  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

posite  of  divergent  bundles  shorter  than  the  radius,  which  have  been 
added  to  one  another  as  though  new  plumes  had  started  from  the  ends 
of  earlier  ones. 

In  most  cases  the  middle  portion  of  the  feldspar  bundles  consists  of 
stouter  crystals  than  the  outer  parts.  It  also  appears  that  the  middle 
portion  is  more  prismatic,  in  certain  cases  somewhat  cuboidal,  the 
outer  parts  becoming  delicately  tabular.  This,  with  the  divergence  in 
position,  explains  the  spread  of  the  outer  part  of  the  sphere.  There  is 
a  great  increase  in  the  number  of  individual  crystals  in  the  outer 
portion  of  the  spherulite,  and  in  some  cases  the  crystals  also  increase 
in  size  in  the  outer  part. 

The  shapes  of  the  crystals  are  due  to  the  flattening  of  the  crystal 
parallel  to  the  second  pinacoid  (oio),  and  its  elongation  parallel  to 
the  crystal  axis  a.  The  outlines  of  the  plates  appear  to  conform  to 
traces  of  several  pinacoids  in  the  zone  of  the  b  axis,  (ooi),  (201), 
(Toi),  (201),  (304),  (203),  not  all  of  these  occurring  together.  It  is 
quite  probable  that  pinacoids  in  the  zone  of  the  c  axis  also  may  be 
developed,  but  they  were  not  recognized. 

Bladed  forms  in  some  cases  prove  to  be  aggregates  of  thin  plates 
not  strictly  parallel  to  one  another  in  the  plane  of  flattening,  so  that 
the  blade  is  curved  and  not  straight  in  the  direction  of  its  longest  axis. 

In  some  spherulites  the  component  crystals  are  prisms  throughout, 
with  no  tabular  flattening.  The  number  of  crystal  prisms  increases 
from  the  center  of  the  spherulite  outward  by  the  development  of  new 
prisms  at  slightly  divergent  angles,  in  arborescent  arrangement. 

The  most  complex  arrangements  are  produced  by  twinning  and 
divergence  combined,  resulting  in  feather-like  aggregates.  Long, 
narrow,  tapering  blades  in  albite  twins  form  a  shaft,  elongated  parallel 
to  the  crystal  axis  a,  on  two  sides  of  which  diverge  at  a  slight  angle  a 
double  set  of  thin  blades,  like  barbs.  These  consist  of  branched 
smaller  blades  or  prisms,  like  barbules,  the  branch  prisms  having 
approximately  the  direction  of  the  crystal  axis  c.  The  two  sets  in 
each  "barb"  are  apparently  related  to  one  another  as  the  halves  of  a 
manebach  twin.  The.  small  prisms  are  composed  of  many  subpar- 
allel  plates  flattened  in  the  plane  of  the  second  pinacoid  (oio) .  These 
correspond  to  barbicels  in  a  feather. 

With  respect  to  the  size  of  the  crystals  it  is  extremely  significant 
that  pure  anorthite  (An)  develops  in  comparatively  large  plates,  5  mm. 
thick  and  20  to  30  mm.  long,  in  a  few  hours,  whereas  the  more  sodic 
the  feldspar  the  smaller  the  individual  crystals  formed  under  almost 
the  same  conditions  of  cooling.  Thus  with  oligoclase  (Ab4An!)  the 
individual  crystals  composing  a  bundle  of  blades  are  considerably 


ANORTHITE   (AN).  8 1 

less  than  o.oi  mm.  thick,  probably  about  o.ooi  mm.,  a  difference  in 
thickness  when  compared  with  anorthite  of  about  5,000  to  i.  This 
as  shown  elsewhere  is  due  to  the  greater  viscosity  of  the  liquid 
feldspars  near  their  solidifying  point  as  they  approach  the  albite  end 
of  the  series. 

Any  comparison  of  the  grain  of  rocks,  that  is,  the  size  of  the  con- 
stituent crystals,  with  a  view  to  determining  the  physical  conditions 
attending  the  solidification  of  the  magma,  must  be  based  in  the  first 
instance  on  a  knowledge  of  the  behavior  of  the  various  rock-making 
minerals  under  similar  physical  conditions,  both  separately  and  in 
combination,  that  is,  in  solution  with  one  another.  The  granu- 
larity of  rocks  is  clearly  a  function  of  the  chemical  composition. 

With  respect  to  the  homogeneity  of  the  crystals  separating  from 
the  liquid,  it  is  observed  that  the  great  part  of  each  crystal  aggregation 
appears  to  be  of  one  composition,  but  that  in  some  cases  a  small 
proportion,  probably  less  than  i  per  cent,  is  different  from  the  bulk  of 
the  feldspar,  both  in  composition  and  habit.  In  one  instance  this 
small  variant  differed  in  composition  but  not  in  habit  from  the  main 
mass  of  crystals. 

In  the  first  case  it  appears  that  crystallization  began  with  feldspar 
richer  in  the  anorthite  molecule  than  the  solution  and  developed 
cuboidal  forms.  These  were  prolonged  into  prismatic  bundles,  the 
prisms  having  the  composition  of  the  main  mass  of  crystals. 

In  the  second  case  the  small  variant  crystallized  toward  the  end  of 
the  crystallization  and  contained  more  albite  molecules  than  the  main 
mass  of  feldspar  crystals.  It  had  the  same  habit  as  the  other  more 
calcic  portion,  and  appears  to  have  crystallized  at  the  same  time  with 
it,  the  crystals  with  different  optical  properties  being  by  the  side  of 
one  another  and  not  in  zonal  relation.  Neither  of  the  feldspars 
represents  the  end  member  of  the  series,  An  or  Ab. 

The  detailed  description  of  the  thin  sections  of  these  laboratory 
preparations  of  lime-soda  feldspars  follows: 

ANORTHITE  (AN). 

(19).  This  aggregation  consists  of  tabular  crystals  3  to  5  mm. 
thick  in  somewhat  radial  arrangement,  and  between  these  are  smaller 
tabular  crystals  in  similar  radial  clusters.  The  clusters  are  twinned 
according  to  the  albite  law  in  lamellae,  o.  15  mm.  thick  and  less.  The 
thinnest  lamellae  are  not  always  continuous  throughout  the  length  of 
a  crystal. 

The  optical  orientation  is  uniform  throughout  the  length  of  each 


82  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

lamellar  section  without  evidence  of  zonal  structure,  proving  that  the 
crystals  are  chemically  homogeneous. 

Fracture  lines  cross  the  crystal  irregularly  and  follow  possible  cleav- 
age planes  to  only  a  limited  extent.  In  some  cases  lamellae  have 
broken  apart  along  the  composition  plane,  which  is  also  the  second 
cleavage  plane  (oio). 

In  several  places  the  albite  twins  have  been  cut  at  right  angles  to 
the  twinning  plane  and  also  at  right  angles  to  one  of  the  optic  axes  of 
the  crystal.  In  one  lamella  it  is  almost  exactly  normal  to  the  plane 
of  section,  in  the  other  very  slightly  inclined.  The  plane  of  the  optic 
axes  in  one  lamella  stands  at  63°  to  the  trace  of  the  twinning 
plane  (oio);  in  the  other  lamella  it  is  62°  30'  approximately.  One 
of  the  optic  axes  of  the  crystal  lies  almost  'parallel  to  the  pinacoid 
(oio).  This  is  the  position  given  it  in  Michel-Levy's  diagram*  for 
anorthite  (An). 

The  crystals  contain  numerous  inclusions  of  colorless,  apparently 
amorphous  substance,  with  a  refraction  higher  than  anorthite.  It 
appears  to  be  iso tropic.  The  outline  is  very  irregular  and  rounded. 
The  shapes  are  curved  and  elongated.  They  contain  gas  bubbles. 
These  inclusions  are  distributed  in  planes,  lines,  and  swarms,  having 
various  directions  with  respect  to  the  feldspar  crystals.  The  arrange- 
ment in  some  cases  suggests  skeleton  forms.  In  some  places  the 
inclusions  are  mostly  gas.  But  the  suggested  feather  structure  bears 
no  fixed  relation  to  the  crystal  orientation  or  the  lamellar  structure 
of  the  anorthite.  It  appears  like  the  structure  of  something  obliter- 
ated by  the  crystallization  of  the  anorthite,  or  in  some  cases  as  though 
the  skeleton  form  were  completely  filled  up  by  anorthite  in  perfect 
orientation.  The  distribution  of  the  inclusions  in  some  instances  is 
such  as  to  suggest  changes  in  the  rate  of  crystallization  of  different 
parts  of  the  crystal.  In  some  places  the  crowding  of  minute  inclusions 
suggests  very  rapid  crystallization.  It  appears  as  faint  cross-banding 
in  the  crystal  shown  in  Plate  I,  from  another  preparation  of  anor- 
thite (An). 

The  glass  is  probably  composed  of  material  in  excess  of  the  propor- 
tions necessary  for  the  anorthite  (CaAl2  SizOg  =  An) .  It  is  not  SiO2 
alone,  for  this  would  have  an  index  of  refraction  lower  than  anorthite. 
It  may  be  a  silicate  of  Al  or  Ca. 

(5oa~b).  This  preparation  is  similar  to  (19).  The  crystals  are 
tabular  parallel  to  the  second  pinacoid  (oio).  The  larger  plates  are 
1.5  mm.  thick.  There  is  multiple  twinning  according  to  the  albite 


*  £tude  sur  la  Determination  des  Feldspaths,  etc.     Paris,  1894.     Plate  vii. 


BYTOWNITE  (ABjAN5).  83 

law,  and  no  evidence  of  variation  in  optical  orientation  or  zonal  struc- 
ture in  any  one  crystal.  The  crystals  are  homogeneous.  In  sections 
cut  at  right  angles  to  an  optic  axis  the  plane  of  the  optic  axes  makes 
an  angle  of  65°  with  the  trace  of  the  second  pinacoid  (oio).  In  some 
sections  there  is  a  remarkable  appearance  of  the  twinned  lamellae. 
They  appear  to  be  faulted  in  bands  across  the  tabular  crystal,  as 
shown  in  Plate  I.  But  there  is  no  evidence  of  dislocation  in  the  out- 
line of  the  crystal  plate ;  in  fact,  there  may  be  continuous  lamellae  on 
both  sides  of  the  apparently  faulted  belt.  Close  inspection  of  twinned 
lamellae  shows  that  the  several  series  of  discordant  belts  do  not  cor- 
respond in  number  or  in  width  of  the  lamellae  composing  them,  so 
that  they  are  not  displaced,  faulted  sections  of  a  large  multiple  twin 
of  feldspar,  but  independent  crystallizations  in  parallel  position. 

The  illustration  shows  a  cross-section  of  tabular  feldspar  cut  at 
right  angles  to  one  optic  axis  and  nearly  at  right  angles  to  the  crys- 
tallographic  axis  c.  The  crystal  is  tabular  parallel  to  the  pinacoid 
(oio) ;  the  belts  of  multiple  twins,  which  have  the  appearance  of  being 
faulted,  extend  at  right  angles  to  (oio).  Their  growth  appears  to 
have  progressed  from  one  side  of  the  tabular  crystal  to  the  other,  for 
they  are  blended  with  a  continuous  lamella  on  one  side  and  exhibit 
a  broken  limiting  line  on  the  other  side,  against  another  continuous 
lamella.  They  may  represent  a  coordinated  set  of  prismatic  feldspar 
crystals,  elongated  parallel  to  the  crystallographic  axis  c,  twinning 
independently  of  one  another,  while  thickening  in  the  direction  of 
the  b  axis. 

BYTOWNITE  (AB^NB). 

(58a~b).  These  sections  are  from  spherulitic  aggregations  of 
twinned  crystals.  The  spherulite  consists  of  radiating  groups  of 
highly  twinned  bladed  crystals  of  feldspar,  which  are  nearly  parallel 
to  one  another  within  one  group.  But  the  different  groups  stand  at 
various  angles  to  one  another.  This  is  shown  in  cross-section  (58b), 
Plate  II.  The  blades  are  not  plane-faced  or  parallel-faced.  They 
curve  somewhat  and  wedge  out  abruptly.  They  vary  in  thickness 
from  o.i  8  mm.  to  about  0.07  mm.  and  less.  The  breadth  of  the 
blades  varies  considerably,  averaging  about  i  mm  The  groups  of 
subparallel  blades  are  from  2  to  5  mm.  in  diameter.  In  length,  as 
shown  in  section  (58*),  Plate  III,  the  blades  are  about  10  mm.  long. 

Upon  magnifying  these  blades  they  are  seen  to  be  highly  complex, 
and  their  outline  quite  irregular.  Cross-sections  exhibit  multiple 
twinning  according  to  the  albite  law,  the  lamellae  being  sharply  defined 
in  some  places  and  indistinct  in  others.  In  thin  section  there  are 


84  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

opaque  lines  between  blades.  These  are  very  irregular  and  appear 
to  be  impurities  introduced  into  spaces  between  the  surfaces  of  blades 
during  the  grinding  of  the  section.  The  surfaces  of  blades  are  com- 
posed of  crystal  faces  at  various  angles  which  are  the  terminations 
of  component  lamellae.  A  blade  so  delimited  in  cross-section  is  trav- 
ersed in  places  by  distinct,  straight  lamellae,  which  extend  without 
interruption  from  one  side  of  the  blade  to  the  other,  as  shown  in 
Plate  II,  On  both  sides  of  such  a  twinned  belt  the  blade  is  composed  of 
shorter  lamellae  which  appear  to  originate  near  the  middle  of  the  blade 
and  run  outward,  their  cross-section  being  like  a  curved  wedge  whose 
apex  is  at  the  middle  of  the  blade.  In  other  places  there  are  parallel- 
edged  lamellae  in  two  bands  standing  at  a  slight  angle  to  each  other. 
These  lamellae  appear  to  blend  or  to  be  interwoven  in  the  middle  of 
the  blade.  They  are  not  twinned  in  some  cases,  but  are  twinned  in 
others.  Each  of  these  lamellae  terminates  at  the  surface  of  the  blade 
as  an  independent  crystal,  so  that  the  surface  consists  of  the  angular 
terminations  of  these  crystals.  In  some  places  they  terminate  in 
a  common  plane.  As  they  do  not  always  exhibit  uniform  optical 
behavior,  they  appear  to  overlie  one  another  in  thin  section  as 
inclined  plates  or  prisms. 

In  longitudinal  section  (58a)  parallel  to  the  rays  of  the  spherulite 
the  long  shafts  of  feldspar  exhibit  very  delicate  feather-like  structure. 
This  is  very  intricate  and  is  often  blended  to  such  an  extent  that  its 
precise  character  is  obscure.  It  appears  differently  according  to  the 
position  in  which  the  groups  of  bladed  crystals  or  aggregations  have 
been  cut  by  the  section.  In  some  positions  of  the  section  the  shaft  of 
a  "feather"  consists  of  long,  narrow,  very  sharply  defined  stripes  of 
albite  twins  (Plate  III),  which  are  clearly  longitudinal  sections  of  the 
well-defined  bands  of  albite  twins  observed  in  cross-sections  of  the 
blades.  This  shaft  tapers  gradually  toward  the  apex.  On  both 
sides  of  this  shaft  are  long,  straight-edged  lamellae,  which  make  an 
angle  of  4°  or  5°  with  the  twinned  shaft,  and  farther  out  toward  the 
apex  an  angle  of  7°.  They  appear  like  barbs  in  a  feather.  In  some 
places,  noticeably  toward  the  apex  of  the  feather,  the  barb-like  parts 
are  crossed  by  delicate  parallel  lines,  like  barbules. 

The  position  of  the  twinned  lamellae  parallel  to  (oio)  and  the  devel- 
opment of  the  pinacoidal  cleavages  parallel  to  (oio)  and  (ooi)  which 
appear  in  cross-sections  (58b)  show  that  the  feather-like  blade  is 
elongated  in  the  direction  of  the  crystal  axis  a,  and  is  broadened  par- 
allel to  the  basal  pinacoid  (ooi).  Some  longitudinal  sections  parallel 
to  the  second  pinacoid  (oio)  show  a  feather-like  arrangement  of  some- 
what curved  branches  or  barbs,  each  composed  of  extremely  thin 


BYTOWNITE  (AB^NJ.).  85 


plates  in  parallel  orientation.  The  plates  are  tabular  in  the  pinacoid 
(oio)  and  are  bounded  by  the  planes  (ooi),  (100),  (201),  and  (101). 
The  angle  at  which  these  barbs  approach  the  central  part  of  the 
feather,  though  somewhat  variable,  is  approximately  65°  in  some 
sections.  This  suggests  the  manebach  twinning  in  the  portions  of 
the  aggregated  blades  from  which  such  longitudinal  sections  are  cut. 

Referring  to  the  illustrations  already  mentioned,  it  will  appear  that 
the  aggregated  blades,  which  in  subparallel  bundles  form  the  rays  of 
the  spherulite,  consist  in  certain  parts  of  long,  flat,  twinned  lamellae, 
elongated  in  the  direction  of  the  crystal  axis  a,  each  lamella  flattened 
parallel  to  the  second  pinacoid  (oio),  the  plane  of  twinning.  On 
both  sides  of  these  twinned  lamellae  are  slightly  inclined,  thin,  flat, 
blade-like  lamellae  (barbs)  which  are  in  double  arrangement  on  each 
side  of  the  shaft,  as  shown  in  cross-section  (Plate  II)  .  In  other  parts  of 
the  composite  blades  these  doubled  barbs  appear  to  be  compounded 
of  thin  plates  flattened  parallel  to  (oio)  (barbules),  the  doubling 
appearing  to  be  due  to  manebach  twinning.  In  the  middle  portion 
of  such  aggregates  there  is  great  confusion  of  detail,  due  to  wedging  of 
crystals  and  overlapping  within  the  thin  section.  Crystallization 
appears  to  have  advanced  from  the  central  portion  of  such  aggregates 
outward,  producing  in  some  places  wedge-shaped  crystals  or  wedge- 
shaped  aggregates  of  parallel  tabular  crystals,  which  may  behave  as 
a  continuous  crystal  within  the  body  of  the  aggregate,  but  may  have 
an  outline  or  surface  corresponding  to  a  parallel  aggregation  of  smaller 
crystals. 

The  optical  behavior  of  these  feldspar  aggregates  indicates  that 
their  substance  is  homogeneous  throughout,  except  in  several  places 
where  the  optical  properties  show  that  feldspar  of  another  composi- 
tion has  crystallized.  These  portions  are  small  in  proportion  to  the 
bulk  of  the  feldspar  crystals.  They  are  in  several  instances  fortu- 
nately cut  by  the  thin  section,  the  two  cleavages  being  almost  exactly 
normal  to  the  section  plane.  The  section  is  almost  perpendicular  to 
(oio)  and  (ooi).  In  these  feldspars  the  acute  bisectrix  is  almost 
exactly  normal  to  the  plane  of  section  ;  the  plane  of  the  optic  axes  is 
parallel  to  the  basal  cleavage  (ooi).  The  acute  bisectrix  is  the  direc- 
tion of  vibration  of  the  fastest  ray,  the  mineral  is  optically  negative, 
and  the  optical  properties  are  those  of  Ab4Ani,  as  shown  in  Michel- 
Levy's  diagram.*  Apparently  the  feldspar  compound  first  crystal- 
lizing was  a  little  richer  in  calcium  than  the  mixture  of  the  solution, 
and  feldspar  of  this  composition  continued  to  grow  until  the  solution 

*  Op.  tit.,  Platen. 


86  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

in  places  reached  the  composition  of  Ab4Ani.  But  there  is  no  evi- 
dence that  the  crystallization  of  the  more  calcic  feldspar  had  ceased 
before  that  of  the  more  sodic  feldspar  began.  On  the  contrary,  they 
appear,  judging  from  the  shape  of  the  aggregates,  to  have  grown 
simultaneously,  but  toward  the  end  of  the  act  of  solidification  of  the 
solution. 

(30)  and  (31).  These  are  thin  sections  from  one  preparation,  which 
is  glassy  below  and  crystalline  above.  (31)  is  from  the  upper  part  of 
the  preparation  at  right  angles  to  the  upper  surface.  It  consists  of 
a  minutely  crystallized  aggregation  with  more  coarsely  crystallized 
parts  composed  of  lath-shaped  and  bladed  crystals,  the  largest  being 
1.8  mm.  long  and  o.  18  mm.  thick,  there  being  all  gradations  in  size 
from  the  largest  to  the  smallest  (Plate  IV).  The  crystals  lie  at  all 
angles,  sometimes  radiating,  but  spherulitic  aggregates  have  not  been 
developed.  In  many  cases  the  crystals  have  rectangular  outlines ;  in 
others  they  taper  at  the  extremities,  or  wedge  out  by  reason  of  the 
interference  of  adjacent  crystals.  Twinning  is  common,  but  some 
are  not  twinned.  The  albite  law  prevails,  and  in  some  crystals  the 
symmetrical  extinction  angles  are  45°.  The  habit  of  the  crystals  is 
not  definitely  determinable,  whether  tabular  plates  or  elongated 
blades.  The  crystals  are  of  different  thicknesses  within  the  thin 
section,  the  plates  or  blades  being  thinner  than  the  section  of  the 
preparation.  For  this  reason  the  Becke  method  of  testing  relative 
refringence  of  adjacent  crystals  is  not  applicable,  as  the  thicker 
parts  of  crystals  in  glass  or  balsam  always  appear  in  higher  relief 
than  the  thinner  parts. 

In  (30),  from  the  middle  of  the  preparation,  just  above  the  glassy 
bottom  half,  there  are  aggregations  of  delicate  tabular  crystals  in  par- 
allel and  subparallel  groups  in  various  angular  positions  (PI.  V).  In 
some  places  the  tabular  crystals  in  cross-section  are  sharply  outlined 
and  straight- edged.  In  most  cases  the  outline  is  indefinite  and  the 
larger  plates  consist  of  multitudes  of  parallel  and  subparallel  plates, 
whose  outline  in  the  plane  of  flattening,  however,  is  often  sharply 
defined.  They  appear  to  be  plates  parallel  to  (oio),  bounded  by  the 
pinacoids  (ooi),  (201),  and  (Toi).  The  angle  between  the  traces  of 
(ooi)  and  (201)  is  about  81°,  and  that  between  the  traces  of  (ooi) 
and  (Toi)  about  52°.  In  some  cases  the  plates  are  nearly  equi- 
dimensional,  in  others  they  are  elongated  into  blades  parallel  to  (ooi) 
or  to  (201).  Owing  to  the  aggregation  of  subparallel  crystals  of 
extreme  thinness,  the  optical  behavior  is  that  of  aggregates,  and 
confused.  In  one  plate  of  a  thicker  crystal  cut  parallel  to  (oio) 


LABRADORITE   (ABjAN.,).  87 

the  extinction  angle,  measured  from  the  cleavage  plane  (OQI),  is 
about  33°.  The  crystals  appear  to  be  alike  and  homogeneous,  hav- 
ing the  composition  A 


lyABRADORlTE  (A 

(6oa~b).  The  preparation  consists  of  radiating  plates  or  blades, 
about  0.05  mm.  to  less  in  width  and  as  much  as  0.7  mm.  long.  Two  to 
five  blades  intersect  at  various  angles,  wedging  out  at  the  point  of 
intersection  (Plate  VI).  Bach  plate  consists  of  two  or  more  twinned 
lamellae.  Between  the  thicker  plates  there  are  more  delicate  crystals 
composed  of  subparallel  plates  and  skeleton  growths  of  extremely 
thin  blades  with  crystal  outline,  probably  the  traces  of  (ooi)  and 
(201)  on  the  second  pinacoid  (oio).  The  more  solid  plates  or  blades 
feather  out  at  the  ends  to  somewhat  divergent  plumes.  There  are 
in  some  cases  branching,  feather-like  forms  in  crystallographic  posi- 
tions suggesting  the  extension  of  a  single  crystal  in  directions  parallel 
to  the  a  and  c,  and  possibly  the  b,  axes,  the  angles  of  branching  being 
about  64°,  and  in  some  cases  about  90°.  Albite  twins  yield  maximum 
symmetrical  extinction  angles  of  37°.  The  crystals  appear  to  be 
homogeneous. 

(6ia~b).  The  preparation  is  glass,  with  an  index  of  refraction 
higher  than  that  of  balsam,  and  feldspar  spherulites  about  10  mm. 
in  diameter.  The  spherulites  are  very  beautiful  aggregations  of 
somewhat  divergent,  plumose  bundles  of  prismatic  crystals  (Plate  VII) 
that  appear  as  distinct  crystals  at  the  surface  of  the  spherulite,  from 
which  they  project  at  slightly  different  lengths-  into  the  surrounding 
glass,  each  prism,  0.003  or  0.004  mm-  m  diameter,  being  terminated 
by  crystal  faces  nearly  equally  inclined  to  the  long  axis  of  the  prism. 
The  component  short  bundles  show  similar  plagiohedral  terminations 
to  the  individual  prisms  composing  them.  These  in  some  cases  are 
flattened  and  blade-like,  and  are  in  subparallel  aggregations,  the 
plates  having  nearly  rectangular  outline.  In  one  part  of  section 
(6ia)  there  are  groups  of  albite  twinned  feather-like  aggregates  similar 
to  those  in  (58b).  The  groups  are  shorter  and  less  parallel,  and  are 
more  curved.  There  are  longitudinal  sections  of  radial  elements  of 
the  spherulites  (Plate  VIII)  with  the  same  feather-like  structure 
observed  in  (s8a).  The  feldspars  of  the  spherulites  appear  to  be 
homogeneous  optically,  and  are  probably  so  chemically. 

(23a~b).  An  aggregation  of  radiating  blades  and  possibly  prisms 
somewhat  spherulitic,  the  radii  being  5  mm.  long  in  some  cases  (Plate 
IX)  .  The  apparently  prismatic  forms  may  be  cross-sections  of  blades 
which  are  recognizable  as  such  in  other  positions.  They  form  dis- 


88  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

tinctly  branching,  curved,  radiating  aggregates,  in  some  cases  exhibit- 
ing albite  twinning.  In  places  these  prismatic,  rod-like  forms  broaden 
out  to  rectangular  cuboidal  shapes,  which  extend  in  short  prismatic 
branches  almost  at  right  angles  to  the  longitudinal  direction  of  the 
long  prisms.  These  cuboidal  crystals  show  to  a  slight  extent  a  cross 
twinning  which  is  in  the  position  for  pericline  twinning.  The  angle 
between  the  traces  of  the  cuboidal  faces  is  nearly  90°,  which  indicates 
that  the  crystals  have  been  cut  parallel  to  the  basal  plane  and  the 
sections  are  bounded  by  the  first  and  second  pinacoids  (100)  (oio). 
The  sections  parallel  to  the  flat  side  of  the  blades  show  an  intricate 
structure  composed  of  parallel  and  subparallel  thin  plates  with  crystal 
outlines  at  several  angles,  the  most  frequent  being  nearly  90°.  These 
aggregates  of  plates  form  bands  that  branch  in  feather-like  structures 
( Plate  X) .  There  are  occasionally  small  rectangular  but  quite  irregu- 
larly outlined  sections,  whose  shape  is  that  of  cuboidal  crystals,  with 
zonal  markings  about  the  center,  which  have  developed  small  pris- 
matic projections  parallel  to  one  axis  (?a),  the  prisms  being  located 
at  the  four  corners  of  the  rectangular  section.  This  corresponds  to 
the  microlitic  crystals  of  feldspar  found  in  volcanic  glasses,  where  the 
projecting  prisms  are  delicate  fibers.  The  feldspar  crystals  in  this 
preparation  appear  to  be  homogeneous. 

(22a"b~c).  The  preparation  was  first  cooled  rapidly  from  a  melted 
condition,  then  heated  again  to  1250°  at  a  maximum.  This  has  had 
a  very  interesting  result,  namely,  two  periods  of  crystallization,  the 
first  rapid,  the  second  slower.  The  main  mass  of  the  preparation 
consists  of  bundles  of  feldspar  fibers^and  delicate  network  of  crossed 
fibers.  The  bundles  of  fibers  are  about  0.015  mm.  wide  and  o.  165  mm. 
long,  and  occur  singly  or  cross  one  another  at  various  angles,  several 
intersecting  in  the  middle.  Single  bundles  have  two  strong  tapering 
fibers  on  the  outside  spreading  slightly.  These  bundles  of  fibers 
extinguish  light  parallel,  or  at  a  small  angle  (7°),  to  the  length  of  the 
fiber.  The  fastest  ray  vibrates  nearly  parallel  to  the  fibers,  which 
appear  to  be  elongated  in  the  direction  of  the  axis  a.  There  is  a  small 
amount  of  isotropic  glass. 

Within  this  mass  are  spherulites  about  10  mm.  in  diameter,  the 
outer  shell,  i  millimeter  thick,  having  a  somewhat  different  appear- 
ance from  the  central  portion.  The  central  part  consists  of  radiating, 
branching  prisms  or  blades,  not  in  straight  rays  but  in  plumose  aggre- 
gations. The  outer  marginal  zone  is  a  blend  of  the  inner  spherulite 
and  surrounding  matrix  of  small  bundles  of  fibers  (Plate  XI),  and  it 
is  evident  that  the  spherulitic  feldspar  crystallization  had  advanced 
in  the  already  crystalline  matrix  by  a  process  of  recrystallization,  the 


ANDESINE-LABRADORITE  (AB^Ni).  89 

small  bundles  of  fibers  losing  their  optical  orientation  and  finally  their 
distinct  outline.  Their  place  is  occupied  by  spherulitic  feldspar  rays 
whose  orientation  is  independent  of  the  position  of  the  former  bundles 
of  fibers.  The  former  position  of  the  bundles  is  shown  in  places  by 
pairs  of  stouter  fibers,  the  outside  members  of  the  bundles  of  fibers ; 
in  other  places  by  clusters  of  minute  inclusions  resembling  air  spaces 
between  a  network  of  fibers.  These  fade  out  in  passing  from  the 
margin  of  the  spherulite  inward  through  the  millimeter-thick  shell. 
They  have  entirely  disappeared  in  the  central  part  of  the  spherulite. 
The  feldspar  of  the  spherulite  is  homogeneous  and  the  small  bundles 
appear  homogeneous.  They  must  have  the  composition  of  the  pre- 
pared mixture.  The  small  angle  of  extinction  of  the  bundles  of  fibers 
is  difficult  to  account  for.  It  behaves  more  like  oligoclase  than 
labradorite. 

(20).  A  branching  spherulitic  aggregation  with  numerous  cavities 
elongated  in  the  direction  of  the  radiating  fibers.  The  section  is  too 
thick  to  show  well  the  microscopic  structure  of  the  parts.  They 
appear  to  be  prisms  and  blades  composed  of  minute  subparallel 
parts.  The  feldspar  appears  to  be  homogeneous. 

ANDESINE-LABRADORITE  (ABjANj. 

(26).  The  section  of  this  preparation  is  extremely  fine-grained 
at  one  end  and  glassy  at  the  other,  the  lower  end  of  the  crucible. 
The  crystals  and  spherulites  become  larger  toward  the  glassy  end  of 
the  section  (Plate  XII).  The  glass  has  lower  refraction  than  balsam. 
There  are  many  opaque  white  lumps,  which  appear  to  be  unmelted 
powder.  They  constitute  5  to  10  per  cent  of  the  mass. 

The  feldspar  crystals  form  fibrous  bundles,  single  and  in  sets,  cross- 
ing one  another  at  various  angles,  spreading  out  into  plumes  and 
spherulitic  groups. 

In  the  central  portion  of  some  fibrous  bundles  there  are  rectangu- 
lar, lath-shaped,  and  block-like  crystals  with  albite  twinning,  which 
exhibit  symmetrical  extinction  angles  of  45°.  These  parts  must  have 
the  composition  of  A^Ang,  at  least.  The  thinner  prisms  or  fibers 
outside  of  these  show  lower  extinction  angles,  about  25°.  If  these 
correspond  to  maxima  in  each  case,  the  fibrous  feldspar  is  about 
AbiAnt.  The  feldspars,  except  the  central  portions,  which  are  com- 
paratively few,  appear  to  be  homogeneous.  Some  of  the  plumose 
aggregates  which  are  albite  twins  are  very  beautiful. 

(27).  This  preparation  consists  of  glass  with  twinned  prisms  of 
feldspar  in  radiating  groups  about  i  mm.  long  which  are  not  properly 
spherulites.  They  are  shown  in  Plate  XIII.  They  are  in  most  cases 


90  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

prisms,  and  not  plates  or  blades.  In  places  these  are  thin  plates  or 
blades  in  twinned  aggregations,  as  in  (58a~b).  The  prisms  are  more 
distinctly  developed  at  the  ends  of  some  of  the  radiating  aggregates, 
where  they  are  distinctly  twinned  according  to  the  albite  law  and 
yield  symmetrical  extinction  angles  of  30°. 

(59a~b)-  This  preparation  is  glass,  with  spherulitic  aggregations 
about  2  mm.  in  diameter  grading  into  smaller  radiating  bundles  cross- 
ing one  another  in  groups  of  2,  3,  4,  and  more,  as  in  other  cases  already 
described.  The  middle  part  of  the  bundles  consists  of  stout  prisms 
passing  into  extremely  thin  fibers.  The  stouter  portion  yields  ex- 
tinction angles  of  30°.  The  feldspars  appear  to  be  homogeneous 
crystals. 

(64a~b).  This  preparation  consists  of  spherulitic  aggregations  simi- 
lar to  (59),  and  also  short  rectangular  prisms  with  almost  square 
cross-section.  They  are  0.04  to  0.07  mm.  long  and  0.007  to  o.oio  mm. 
wide.  The  forms  are  similar  to  the  lath-shaped  feldspar  microlites 
common  in  andesites.  They  are  in  some  cases  twinned,  in  others  not. 
They  yield  extinction  angles  of  30°. 

ANDESINE  (AB-jANj). 

(54a~b).  This  preparation  is  extremely  minutely  crystallized.  The 
main  mass  appears  to  be  holocrystalline,  composed  of  flake-like 
microlites  of  feldspar  overlapping  one  another  at  all  angles,  so  as  to 
produce  weak  double  refraction.  The  crystals  are  larger  in  patches 
and  in  shells  about  isotropic  spaces,  as  though  the  crystallization  was 
coarse  about  small  spaces,  like  the  walls  of  cavities  of  geodes.  The 
feldspar  crystals  project  into  the  spaces.  But  these  spaces  are 
filled  with  colorless  isotropic  material  with  refraction  considerably 
lower  than  balsam,  presumably  glass.  It  amounts  to  several  per  cent 
of  the  whole.  This  residual  glass  probably  has  a  different  composi- 
tion from  the  feldspar  mass,  otherwise  it  should  not  have  solidified  as 
glass,  for  the  larger  crystallization  of  the  feldspar  in  juxtaposition  with 
it  indicates  that  the  controlling  conditions  became  more  favorable  to 
the  crystallization  of  the  feldspar. 

(66a~b) .  The  small  thin  sections  of  this  preparation  are  holocrystal- 
line, without  glass.  The  preparation  consists  of  crossed  bundles  of 
prisms  and  blades,  without  true  spherulites.  The  bundles  vary  in 
width  from  o.o i  mm.  to  less  and  in  length  from  o. 5  mm.  to  less.  Some 
of  the  crystals  are  in  albite  twins,  others  not.  While  the  prismatic 
sections  exhibit  nearly  parallel  extinction  in  all  cases,  the  long  axis  of 
the  prism  being  the  direction  of  vibration  of  the  fastest  ray,  and  appear 


OUGOCLASE-ANDESINE  (AB3AN,). 


to  be  alike  in  composition,  there  are  rectangular  sections,  which  at 
first  appear  to  be  cross-sections  of  square  prisms  with  hollow  centers, 
but  are  nearly  equidimensional  crystals  which  in  some  cases  have 
prismatic  prolongations.  These  rectangular  crystals  have  rectangular 
spaces  at  the  center,  are  twinned,  and  exhibit  symmetrical  extinction 
angles  of  about  30°.  As  in  other  preparations  of  the  more  sodic 
feldspars,  there  are  comparatively  few  small  crystals  of  more  calcic 
feldspar,  approximately  AbiAni,  which  began  to  form  in  cuboidal 
shapes,  but  were  followed  by  the  crystallization  of  the  bulk  of  the 
mixture  in  feldspars  of  the  average  composition. 


(21).  Colorless  glass,  without  crystals  in  the  thin  section  studied 
microscopically. 

(32).  Colorless  glass,  with  feldspar  microlites  and  aggregates  in  the 
form  of  bundles  about  o.  i  mm.  long  and  in  crossed  bundles  and  to 
some  extent  in  spherulitic  arrangements.  The  isolated  microlites 


FIG.  25. — Microlites  of  oligoclase-andesine  (AbgAnj). 

are  instructive  both  on  account  of  the  exhibition  of  the  habit  of  the 
feldspar  crystals  in  these  preparations  and  also  as  an  evidence  of  the 
changes  in  habit  during  the  short  period  of  their  growth.  There  are 
two  types  of  microscopic  crystals,  one  tabular,  the  other  prismatic. 
These  occur  near  one  another  intermingled  in  the  glass.  They  are 
0.03  mm.  long  and  smaller. 

In  many  cases  there  appears  to  be  a  nucleus  of  feldspar  within 
feldspar;  in  some  of  these  there  are  also  small,  irregularly  shaped, 
colorless  grains  with  rather  strong  index  of  refraction  whose  compo- 
sition is  not  determinable.  These  are  extremely  minute  and  not 
abundant. 

The  feldspar  nucleus  exhibits  stronger  refraction  than  the  marginal 
feldspar,  but  the  direction  of  extinction  is  the  same  in  both  parts, 
proving  like  optical  orientation  and  showing  that  the  central  part  of 
the  microlite  is  thicker  than  the  margin  and  of  the  same  composition. 
The  initial  crystal  in  these  cases  is  thicker,  that  is,  the  crystallization 


92  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

was  somewhat  more  uniform  from  all  sides,  at  the  beginning,  but 
became  more  rapid,  developing  very  thin  plates  or  prisms  outside 
the  nucleus. 

These  are  illustrated  in  fig.  25.  In  one  (a)  several  minute  grains 
form  a  nucleus  of  a  tabular  feldspar  microlite,  whose  form  indicates 
that  it  is  tabular  parallel  to  the  pinacoid  (oio)  and  is  bounded  by 
(Toi)  and  (201).  The  central  part  of  this  plate  is  thicker  than  the 
margin,  and  thins  out  in  irregularly  placed  rays,  like  spurs  and  gulches 
leading  from  a  mesa  to  a  plain.  The  microlite  is  double,  consisting  of 
two  thin  plates  in  parallel  position.  A  cross-section  of  such  a  double- 
plated  microlite  is  shown  in  (b). 

In  (c)  the  central  part  is  a  flattened  prism  or  blade,  passing  at  the 
extremities  into  fibers  or  needle-like  prisms  and  then  into  a  thin  plate 
completely  surrounding  the  nucleus.  In  (d)  the  thin  plates  at  both 
ends  of  the  blade  do  not  unite.  The  shape  of  the  plate  shows  only  a 
center  of  symmetry;  diagonally  opposite  corners  are  sharp  angles  a 
little  less  than  90°  (foi)  and  (201).  The  other  corners  are  rounded  or 
formed  by  two  obtuse  angles.  The  outline  corresponds  to  the  traces 
of  the  basal  pinacoid  (ooi)  and  the  two  pinacoids  of  the  second  kind 
(201)  and  (ioi)  on  the  second  pinacoid  (oio).  The  crystals  are 
elongated  in  the  direction  of  the  crystal  axis,  a,  and  flattened  parallel 
to  the  second  pinacoid  (oio).  In  (e)  a  small  rectangular  prismatic 
crystal  is  enlarged  to  a  more  elongated  form  with  fibrous  termina- 
tion. In  (/)  a  squarish  form  with  rectangular  projections  is  extended 
in  opposite  directions  as  straight  fibers.  There  are  narrow  prismatic 
forms  with  needle-like  extensions  at  the  ends.  These  are  isolated  or 
more  often  grouped  in  subparallel  bundles  and  in  almost  parallel 
lines,  as  in  (g) .  These  microscopic  crystals  vary  in  size  from  a  length 
of  0.64  mm.  to  less.  They  are  so  small  that  their  optical  characters 
can  not  be  used  to  determine  their  chemical  composition.  They 
appear  to  be  alike  and  homogeneous,  and  may  be  assumed  to  have 
the  composition  of  the  preparation  Ab3An,. 

(28).  This  preparation  is  colorless  glass,  with  feldspar  crystals  in 
rods  or  prisms  about  0.08  mm.  long  and  0.005  mm.  wide.  A  few  of 
these  are  isolated ;  most  of  them  are  crossed  groups  of  two  or  more,  or 
form  radiating  aggregates.  Relatively  few  crystals  have  the  shape  of 
blades  or  plates.  The  prisms  are  frequently  double,  joined  in  the 
middle  like  an  H.  The  central  connecting  part  sometimes  occupies 
a  small  portion  of  the  length  of  the  double  prism,  sometimes  a  large 
portion.  The  prisms  are  grouped  in  subparallel  bundles  as  in  prepara- 
tion (32).  The  extinction  angle  is  almost  zero  in  nearly  all  bundles 
of  prisms.  The  feldspars  appear  to  be  homogeneous,  and  must  have 


OLIGOCLASE-ANDESINE  (ABjANj.  93 

the  same  composition  as  the  preparation,  since  the  mass  is  holocrystal- 
line  in  places. 

(25a).  The  preparation  consists  of  glass  with  spherulitic  aggre- 
gations. These  consist  of  bundles  about  0.4  mm.  long,  of  prismatic 
crystals,  which  are  definite  in  the  middle  and  spread  out  to  plumose 
forms  at  both  ends,  merging  with  others  in  crossed  position  to  form 
spherulites  in  favorable  positions  (Plate  XIV). 

In  the  more  spherulitic  groups  the  terminal  parts  are  made  up  of 
the  most  delicate  fiber-like  crystals.  In  other  parts  of  the  preparation 
the  sheaf-like  bundles  are  composed  of  more  distinct,  thicker  prisms, 
which  are  somewhat  broader  in  one  direction  than  in  another;  that 
is,  they  are  somewhat  blade-shaped.  In  some  cases  these  are  twinned 
according  to  the  albite  law.  In  the  central  parts  of  some  of  the 
bundles  there  is  a  rectangularly  jointed  development  of  the  stouter 
feldspar  crystals  which  are  continuous  with  the  more  prismatic 
crystals  forming  the  main  part  of  the  bundles. 

The  optical  behavior  of  those  rectangular  parts  which  show  albite 
twinning  corresponds  to  that  of  AbiAni, there  being  symmetrical  ex- 
tinction angles  of  30°.  In  the  long  prisms  the  angle  of  extinction  is 
small,  and  the  length  of  the  prisms  is  the  direction  of  vibration  of  the 
fastest  ray.  This  corresponds  to  the  optical  behavior  of  a  more  sodic 
feldspar  with  a  prismatic  development  parallel  to  the  crystal  axis  a. 
In  parts  of  the  preparation  the  feldspars  have  crystallized  in  thin 
tabular  plates  parallel  to  the  second  pinacoid  (oio). 

(24).  This  preparation  is  similar  to  (25a).  It  is  partly  glass, 
partly  spherulitic  aggregations  of  bundles  of  prismatic  crystals, 
spreading  out  at  the  ends.  The  central  portions  of  some  aggrega- 
tions are  of  stout  prisms  and  rectangular  groups  with  symmetrical 
extinction  angles  of  30°  (Plate  XV).  As  in  preparation  (25a)  there 
was  a  first  crystallization  of  feldspar  with  higher  content  in  anor- 
thite  molecules  than  the  average  of  the  mixture.  This  formed  a 
small  fraction,  probably  less  than  i  per  cent  of  the  whole.  The 
principal  crystallization  appears  to  be  homogeneous  and  must  have 
essentially  the  composition  of  the  mixture  Ab3 Ani . 

The  outline  of  the  bladed  crystals  in  the  plane  (oio),  judging  from 
the  optical  orientation,  and  the  elongation  of  the  blades  parallel  to  a, 
is  determined  by  the  pinacoids  of  the  second  kind  (201),  (304)  in 
some  cases  furnishing  an  angle  of  about  80°,  the  angle  between  the 
two  being  nearly  bisected  by  the  trace  of  the  third  pinacoid  (ooi). 
In  other  cases  the  blades  appear  to  be  bounded  by  (201)  and  (203), 
with  the  apex  angle  truncated  by  (201).  Such  plates  are  nearly 


94  ISOMORPHISM  AND  THERMAL  PROPERTIES  OF  FELDSPARS. 

normal  to  the  optical  bisectrix  y,  and  the  plane  of  the  optic  axes  is 
almost  parallel  to  the  direction  of  elongation  of  the  blade. 

(67a~b).  This  preparation  consists  of  spherulites  about  2  mm.  in 
diameter,  and  some  interstitial  glass.  The  spherulites  are  irregular 
in  outline,  due  to  mutual  interference  (Plate  XVI).  They  consist  of 
radiating  sectors  made  up  of  rather  distinct  prisms,  which  start  at 
the  center  of  the  spherulite  as  stout  prisms  and  become  innumerable, 
slightly  diverging  prisms  which  terminate  at  different  lengths  as 
distinct  prisms  terminated  by  a  pinacoid  almost  at  right  angles  to  the 
axis  of  the  prism,  probably  (201)  .  In  other  cases  they  are  terminated 
by  two  pinacoids  making  an  acute  angle.  In  places  the  terminations 
are  somewhat  rounded.  The  crystals  in  these  spherulites  are  homo- 
genous and  correspond  to  the  composition  of  the  mixture. 


(29).  The  preparation  was  heated  to  a  temperature  of  about 
1400°  and  allowed  to  cool  for  15  hours  to  about  425°.  The  resulting 
solid  is  a  glass  with  abundant  crystals  of  feldspar  in  crossed  bun- 
dles of  blades  or  plates,  and  spherulite-like  radiating  aggrega- 
tions (Plate  XVII).  There  are  numerous  rounded  and  subangular 
grains  of  colorless  quartz  in  the  glass  which  have  no  definite  relation 
to  the  feldspar  crystals,  and  appear  to  be  undissolved  fragments  of 
quartz  used  in  compounding  the  preparation.  There  are  also  small 
lumps  of  white  aggregates,  probably  undissolved  powder,  which  in 
many  cases  form  centers  of  spherulitic  crystallization,  that  is,  they 
become  points  at  which  feldspar  crystallization  began. 

The  colorless  glass  has  an  index  of  refraction  noticeably  lower  than 
that  of  Canada  balsam,  1.5393.*  The  length  of  the  feldspar  bundles 
and  diameter  of  the  spherulitic  aggregates  is  about  0.2  mm.,  the  width 
of  the  bundles  about  o.oi  mm.  The  narrow  cross-sections  of  the 
bundles  of  feldspar,  which  has  a  higher  refraction  than  the  glass, 
exhibit  low  extinction  angles,  nearly  parallel  to  the  longitudinal 
direction.  The  bundles  are  clearly  composed  of  numerous  parallel 
or  approximately  parallel  individuals,  which  spread  out  fan-like  or 
plumose  at  the  extremities.  In  other  positions  these  aggregates  are 
seen  to  be  relatively  broad,  blade-like,  or  tabular,  and  made  up  of 
subparallel  plates  of  extreme  thinness.  This  is  shown  by  numerous 
crystal  edges  almost  parallel  to  one  another,  and  also  by  the  com- 
posite character  of  the  interference  phenomena  with  crossed  nicols, 
the  crystal  blade  being  mottled  instead  of  uniformly  dark  in  the 


*  1.5393  is  the  index  of  refraction  of  the  balsam  used  by  the  U.  S.  Geological 
Survey,  as  determined  by  Dr.  J.  E.  Wolff,  in  1896. 


AB^N!).  95 

position  of  extinction.  The  interference  figure  in  convergent  light 
shows  a  bisectrix  normal  to  the  plane  of  the  bladed  crystal  bundle, 
the  bisectrix  being  the  direction  of  vibration  of  the  ray  having  the 
greatest  index  of  refraction,  ?.  The  angle  between  the  optic  axes  is 
larger,  and  the  plane  of  the  optic  axes  lies  in  the  longer  diameter  of  the 
crystal  blade.  This  is  the  optical  orientation  of  Ab4Ani*  when  the 
feldspar  plate  is  tabular  parallel  to  the  second  pinacoid  (oio),  the 
common  case. 

The  angles  between  the  crystal  edges  of  the  thin  plates,  the  position 
of  the  plane  of  optic  axes,  and  the  generally  low  extinction  angles  ex- 
hibited by  longitudinal  cross-sections  of  the  bundles  show  that  the 
feldspar  crystals  are  tabular  parallel  to  (oio)  and  elongated  parallel 
to  the  crystal  axis  a,  and  are  probably  bounded  by  the  planes  (201) 
and  (304)  or  by  (201)  and  (203). 


*  Michel-L£vy,  A.,  op.  cit.,  Plate  in. 


PLATE  I 

CRYSTALS  OF  ANORTHITE  SHOWING  THE  APPEARANCE  OF  DISLOCATION 
IN  THE  TWINNED  LAMELLA.         (50b)  X  57. 


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srv  r> 


PLATE 


TANGENTIAL  SECTION  OF  SPHERULITE  OF  BYTOWNITE  SHOWING  TWINNING 
IN   FEATHER-LIKE   BLADES.         ABiANs  <58b)  X  55. 


k          '^  ^ 
Bk  M 


PLATE 


RADIAL  SECTION  OF  SPHERULITE  OF  BYTOWNITE.        FEATHER-LIKE  AGGREGATES 
OF  TWINNED  CRYSTALS.         ABiAN*  (58»)  X  50. 


PLATE  IV 

CRYSTALS  OF  BYTOWNITE  FROM  TOP  OF  CRUCIBLE. 
ABjANs  '>31/  X  50. 


PLATE  V 

TABULAR  CRYSTALS  OF  BYTOWNITE  FROM   MIDDLE  OF  CRUCIBLE. 
AB.AN.,  (30)  X  50. 


PLATE  VI 

TWINNED  BLADED  CRYSTALS  OF  LABRADORITE. 
ABiAN2  (60)  X  56. 


' 


v 

**'' 


PLATE  VII 

SPHERULITE  OF  PLUMOSE   BUNDLES  OF  PRISMATIC  CRYSTALS  OF  LABRADORITE. 
ABiAN2  (6la^  X  49. 


PLATE  VIII 

SPHERULITE  OF  FEATHER-LIKE  AGGREGATES  OF  CRYSTALS  OF  LABRADORITE. 
ABiAN-..  (61»)  X  49. 


PLATE  IX 

RADIATING  AGGREGATES  OF  BLADES  AND  PRISMS  OF  LABRADORITE. 
ABiAN2  (23a)  X  49 


PLATE  X 

BLADES  COMPOSED  OF  PARALLEL  AND  SUB-PARALLEL  THIN   PLATES  OF  LABRADORITE. 

ABtAN2  (23b)  X  49. 


PLATE  XI 

MARGIN  OF  SPHERULITE  OF  LABRADORITE  BLENDING  WITH   MICROSCOPIC 
FIBERS  OF  SAME   MINERAL.         ABiANa  (22b)  X  55. 


PLATE  XII 

FIBROUS  BUNDLES  AND  SPHERULITES  OF  ANDESINE-LABRADORITE. 
ABiAN!  (26)  X  55. 


PLATE  XIII 

ARBORESCENT  AND  FEATHER-LIKE  AGGREGATIONS  OF  PRISMATIC  AND  BLADED 
CRYSTALS  OF  ANDESINE-LABRADORITE.         ABxAN,   (27)  X  50. 


PLATE  XIV 

SPHERULITES  OF  OLIGOCLASE-ANDESINE  IN  GLASS. 
ABaANi  <25&)  X  50. 


PLATE  XV 

SPHERULITES  OF  OLIGOCLASE-ANDESiNE  IN  GLASS. 
AB3ANi  (24)  X  70. 


PLATE  XVI 

SPHERULITES  OF  OLIGOCLASE-ANDESINE. 
AB.iANi  (67a)  X  50. 


GLASS  W! 


GGREGATES  OF  BLA: 
AB4AN,   (29>  X  55. 


*  rv 


PLATE  XVII 


GLASS  WITH  SPHERULITIC  AGGREGATES  OF  BLADES  OR   PLATES  OF  OLIGOCLASE. 
AB.AN,   (29^  X  55. 


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PLATE  XVII! 


A  SPECIMEN  OF   MITCHELL  COUNTY  ALBITE  BEFORE   HEATING. 
(33)  X  20. 


PLATE  XIX 

A  CLEAVAGE  CRACK  (VERTICAL)  SHOWING  INCIPIENT  MELTING 
AFTER   FOUR   HOURS  AT  1125°.         (51»)   X  600. 


PLATE  XX 

NATURAL  ALBITE  AFTER   HEATING  TO  1200°.       MELTED  AREAS  ARE   DARK. 
(34)  X  40. 


PLATE  XXI 

NATURAL  ALBITE  AFTER  THIRTY-FIVE  MINUTES'   EXPOSURE  AT  1206°. 
MELTED   PORTIONS  ARE   DARK.          (37)   X  40. 


PLATE  XXII 

NATURAL  ALBITE  AFTER  THIRTY  MINUTES'   EXPOSURE  AT  1225°. 
UNMELTED  PORTIONS  ARE   BRIGHT.         (39)  X  40. 


PLATE  XXIII 

NATURAL  ALBITE  AFTER  THIRTY  MINUTES'  EXPOSURE  AT  1247°. 
UNMELTED  PORTIONS  ARE  BRIGHT.         (36)  X  40. 


PLATE  XXIV 

A  CRYSTAL  OF  NATURAL  ALBITE  BENT  AT  1200°  UNDER  LOAD, 
THE  DARK  PORTIONS  HAVE  MELTED,        (41)  X  10, 


PLATE  XXV 

ORTHOCLASE  FRAGMENT   BENT  AT  1200°  UNDER   LOAD.      A  CLEAVAGE  CRACK  (DOTTED)   HAS  RETAINED 
ITS  ORIENTATION,  ALTHOUGH  THE  FRAGMENT  IS  MELTED  THROUGH.         (46)  X  20. 


PLATE  XXVI 

ALBITE  FRAGMENT  BENT  AT  1200°  UNDER   LOAD. 
(45)  X  40. 


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